Reshaping and pivot tables#

Reshaping by pivoting DataFrame objects#

../_images/reshaping_pivot.png

Data is often stored in so-called “stacked” or “record” format:

In [1]: import pandas._testing as tm

In [2]: def unpivot(frame):
   ...:     N, K = frame.shape
   ...:     data = {
   ...:         "value": frame.to_numpy().ravel("F"),
   ...:         "variable": np.asarray(frame.columns).repeat(N),
   ...:         "date": np.tile(np.asarray(frame.index), K),
   ...:     }
   ...:     return pd.DataFrame(data, columns=["date", "variable", "value"])
   ...: 

In [3]: df = unpivot(tm.makeTimeDataFrame(3))

In [4]: df
Out[4]: 
         date variable     value
0  2000-01-03        A  0.469112
1  2000-01-04        A -0.282863
2  2000-01-05        A -1.509059
3  2000-01-03        B -1.135632
4  2000-01-04        B  1.212112
5  2000-01-05        B -0.173215
6  2000-01-03        C  0.119209
7  2000-01-04        C -1.044236
8  2000-01-05        C -0.861849
9  2000-01-03        D -2.104569
10 2000-01-04        D -0.494929
11 2000-01-05        D  1.071804

To select out everything for variable A we could do:

In [5]: filtered = df[df["variable"] == "A"]

In [6]: filtered
Out[6]: 
        date variable     value
0 2000-01-03        A  0.469112
1 2000-01-04        A -0.282863
2 2000-01-05        A -1.509059

But suppose we wish to do time series operations with the variables. A better representation would be where the columns are the unique variables and an index of dates identifies individual observations. To reshape the data into this form, we use the DataFrame.pivot() method (also implemented as a top level function pivot()):

In [7]: pivoted = df.pivot(index="date", columns="variable", values="value")

In [8]: pivoted
Out[8]: 
variable           A         B         C         D
date                                              
2000-01-03  0.469112 -1.135632  0.119209 -2.104569
2000-01-04 -0.282863  1.212112 -1.044236 -0.494929
2000-01-05 -1.509059 -0.173215 -0.861849  1.071804

If the values argument is omitted, and the input DataFrame has more than one column of values which are not used as column or index inputs to pivot(), then the resulting “pivoted” DataFrame will have hierarchical columns whose topmost level indicates the respective value column:

In [9]: df["value2"] = df["value"] * 2

In [10]: pivoted = df.pivot(index="date", columns="variable")

In [11]: pivoted
Out[11]: 
               value                      ...    value2                    
variable           A         B         C  ...         B         C         D
date                                      ...                              
2000-01-03  0.469112 -1.135632  0.119209  ... -2.271265  0.238417 -4.209138
2000-01-04 -0.282863  1.212112 -1.044236  ...  2.424224 -2.088472 -0.989859
2000-01-05 -1.509059 -0.173215 -0.861849  ... -0.346429 -1.723698  2.143608

[3 rows x 8 columns]

You can then select subsets from the pivoted DataFrame:

In [12]: pivoted["value2"]
Out[12]: 
variable           A         B         C         D
date                                              
2000-01-03  0.938225 -2.271265  0.238417 -4.209138
2000-01-04 -0.565727  2.424224 -2.088472 -0.989859
2000-01-05 -3.018117 -0.346429 -1.723698  2.143608

Note that this returns a view on the underlying data in the case where the data are homogeneously-typed.

Note

pivot() will error with a ValueError: Index contains duplicate entries, cannot reshape if the index/column pair is not unique. In this case, consider using pivot_table() which is a generalization of pivot that can handle duplicate values for one index/column pair.

Reshaping by stacking and unstacking#

../_images/reshaping_stack.png

Closely related to the pivot() method are the related stack() and unstack() methods available on Series and DataFrame. These methods are designed to work together with MultiIndex objects (see the section on hierarchical indexing). Here are essentially what these methods do:

  • stack(): “pivot” a level of the (possibly hierarchical) column labels, returning a DataFrame with an index with a new inner-most level of row labels.

  • unstack(): (inverse operation of stack()) “pivot” a level of the (possibly hierarchical) row index to the column axis, producing a reshaped DataFrame with a new inner-most level of column labels.

../_images/reshaping_unstack.png

The clearest way to explain is by example. Let’s take a prior example data set from the hierarchical indexing section:

In [13]: tuples = list(
   ....:     zip(
   ....:         *[
   ....:             ["bar", "bar", "baz", "baz", "foo", "foo", "qux", "qux"],
   ....:             ["one", "two", "one", "two", "one", "two", "one", "two"],
   ....:         ]
   ....:     )
   ....: )
   ....: 

In [14]: index = pd.MultiIndex.from_tuples(tuples, names=["first", "second"])

In [15]: df = pd.DataFrame(np.random.randn(8, 2), index=index, columns=["A", "B"])

In [16]: df2 = df[:4]

In [17]: df2
Out[17]: 
                     A         B
first second                    
bar   one     0.721555 -0.706771
      two    -1.039575  0.271860
baz   one    -0.424972  0.567020
      two     0.276232 -1.087401

The stack() function “compresses” a level in the DataFrame columns to produce either:

If the columns have a MultiIndex, you can choose which level to stack. The stacked level becomes the new lowest level in a MultiIndex on the columns:

In [18]: stacked = df2.stack()

In [19]: stacked
Out[19]: 
first  second   
bar    one     A    0.721555
               B   -0.706771
       two     A   -1.039575
               B    0.271860
baz    one     A   -0.424972
               B    0.567020
       two     A    0.276232
               B   -1.087401
dtype: float64

With a “stacked” DataFrame or Series (having a MultiIndex as the index), the inverse operation of stack() is unstack(), which by default unstacks the last level:

In [20]: stacked.unstack()
Out[20]: 
                     A         B
first second                    
bar   one     0.721555 -0.706771
      two    -1.039575  0.271860
baz   one    -0.424972  0.567020
      two     0.276232 -1.087401

In [21]: stacked.unstack(1)
Out[21]: 
second        one       two
first                      
bar   A  0.721555 -1.039575
      B -0.706771  0.271860
baz   A -0.424972  0.276232
      B  0.567020 -1.087401

In [22]: stacked.unstack(0)
Out[22]: 
first          bar       baz
second                      
one    A  0.721555 -0.424972
       B -0.706771  0.567020
two    A -1.039575  0.276232
       B  0.271860 -1.087401
../_images/reshaping_unstack_1.png

If the indexes have names, you can use the level names instead of specifying the level numbers:

In [23]: stacked.unstack("second")
Out[23]: 
second        one       two
first                      
bar   A  0.721555 -1.039575
      B -0.706771  0.271860
baz   A -0.424972  0.276232
      B  0.567020 -1.087401
../_images/reshaping_unstack_0.png

Notice that the stack() and unstack() methods implicitly sort the index levels involved. Hence a call to stack() and then unstack(), or vice versa, will result in a sorted copy of the original DataFrame or Series:

In [24]: index = pd.MultiIndex.from_product([[2, 1], ["a", "b"]])

In [25]: df = pd.DataFrame(np.random.randn(4), index=index, columns=["A"])

In [26]: df
Out[26]: 
            A
2 a -0.370647
  b -1.157892
1 a -1.344312
  b  0.844885

In [27]: all(df.unstack().stack() == df.sort_index())
Out[27]: True

The above code will raise a TypeError if the call to sort_index() is removed.

Multiple levels#

You may also stack or unstack more than one level at a time by passing a list of levels, in which case the end result is as if each level in the list were processed individually.

In [28]: columns = pd.MultiIndex.from_tuples(
   ....:     [
   ....:         ("A", "cat", "long"),
   ....:         ("B", "cat", "long"),
   ....:         ("A", "dog", "short"),
   ....:         ("B", "dog", "short"),
   ....:     ],
   ....:     names=["exp", "animal", "hair_length"],
   ....: )
   ....: 

In [29]: df = pd.DataFrame(np.random.randn(4, 4), columns=columns)

In [30]: df
Out[30]: 
exp                 A         B         A         B
animal            cat       cat       dog       dog
hair_length      long      long     short     short
0            1.075770 -0.109050  1.643563 -1.469388
1            0.357021 -0.674600 -1.776904 -0.968914
2           -1.294524  0.413738  0.276662 -0.472035
3           -0.013960 -0.362543 -0.006154 -0.923061

In [31]: df.stack(level=["animal", "hair_length"])
Out[31]: 
exp                          A         B
  animal hair_length                    
0 cat    long         1.075770 -0.109050
  dog    short        1.643563 -1.469388
1 cat    long         0.357021 -0.674600
  dog    short       -1.776904 -0.968914
2 cat    long        -1.294524  0.413738
  dog    short        0.276662 -0.472035
3 cat    long        -0.013960 -0.362543
  dog    short       -0.006154 -0.923061

The list of levels can contain either level names or level numbers (but not a mixture of the two).

# df.stack(level=['animal', 'hair_length'])
# from above is equivalent to:
In [32]: df.stack(level=[1, 2])
Out[32]: 
exp                          A         B
  animal hair_length                    
0 cat    long         1.075770 -0.109050
  dog    short        1.643563 -1.469388
1 cat    long         0.357021 -0.674600
  dog    short       -1.776904 -0.968914
2 cat    long        -1.294524  0.413738
  dog    short        0.276662 -0.472035
3 cat    long        -0.013960 -0.362543
  dog    short       -0.006154 -0.923061

Missing data#

These functions are intelligent about handling missing data and do not expect each subgroup within the hierarchical index to have the same set of labels. They also can handle the index being unsorted (but you can make it sorted by calling sort_index(), of course). Here is a more complex example:

In [33]: columns = pd.MultiIndex.from_tuples(
   ....:     [
   ....:         ("A", "cat"),
   ....:         ("B", "dog"),
   ....:         ("B", "cat"),
   ....:         ("A", "dog"),
   ....:     ],
   ....:     names=["exp", "animal"],
   ....: )
   ....: 

In [34]: index = pd.MultiIndex.from_product(
   ....:     [("bar", "baz", "foo", "qux"), ("one", "two")], names=["first", "second"]
   ....: )
   ....: 

In [35]: df = pd.DataFrame(np.random.randn(8, 4), index=index, columns=columns)

In [36]: df2 = df.iloc[[0, 1, 2, 4, 5, 7]]

In [37]: df2
Out[37]: 
exp                  A         B                   A
animal             cat       dog       cat       dog
first second                                        
bar   one     0.895717  0.805244 -1.206412  2.565646
      two     1.431256  1.340309 -1.170299 -0.226169
baz   one     0.410835  0.813850  0.132003 -0.827317
foo   one    -1.413681  1.607920  1.024180  0.569605
      two     0.875906 -2.211372  0.974466 -2.006747
qux   two    -1.226825  0.769804 -1.281247 -0.727707

As mentioned above, stack() can be called with a level argument to select which level in the columns to stack:

In [38]: df2.stack("exp")
Out[38]: 
animal                 cat       dog
first second exp                    
bar   one    A    0.895717  2.565646
             B   -1.206412  0.805244
      two    A    1.431256 -0.226169
             B   -1.170299  1.340309
baz   one    A    0.410835 -0.827317
             B    0.132003  0.813850
foo   one    A   -1.413681  0.569605
             B    1.024180  1.607920
      two    A    0.875906 -2.006747
             B    0.974466 -2.211372
qux   two    A   -1.226825 -0.727707
             B   -1.281247  0.769804

In [39]: df2.stack("animal")
Out[39]: 
exp                         A         B
first second animal                    
bar   one    cat     0.895717 -1.206412
             dog     2.565646  0.805244
      two    cat     1.431256 -1.170299
             dog    -0.226169  1.340309
baz   one    cat     0.410835  0.132003
             dog    -0.827317  0.813850
foo   one    cat    -1.413681  1.024180
             dog     0.569605  1.607920
      two    cat     0.875906  0.974466
             dog    -2.006747 -2.211372
qux   two    cat    -1.226825 -1.281247
             dog    -0.727707  0.769804

Unstacking can result in missing values if subgroups do not have the same set of labels. By default, missing values will be replaced with the default fill value for that data type, NaN for float, NaT for datetimelike, etc. For integer types, by default data will converted to float and missing values will be set to NaN.

In [40]: df3 = df.iloc[[0, 1, 4, 7], [1, 2]]

In [41]: df3
Out[41]: 
exp                  B          
animal             dog       cat
first second                    
bar   one     0.805244 -1.206412
      two     1.340309 -1.170299
foo   one     1.607920  1.024180
qux   two     0.769804 -1.281247

In [42]: df3.unstack()
Out[42]: 
exp            B                              
animal       dog                 cat          
second       one       two       one       two
first                                         
bar     0.805244  1.340309 -1.206412 -1.170299
foo     1.607920       NaN  1.024180       NaN
qux          NaN  0.769804       NaN -1.281247

Alternatively, unstack takes an optional fill_value argument, for specifying the value of missing data.

In [43]: df3.unstack(fill_value=-1e9)
Out[43]: 
exp                B                                          
animal           dog                         cat              
second           one           two           one           two
first                                                         
bar     8.052440e-01  1.340309e+00 -1.206412e+00 -1.170299e+00
foo     1.607920e+00 -1.000000e+09  1.024180e+00 -1.000000e+09
qux    -1.000000e+09  7.698036e-01 -1.000000e+09 -1.281247e+00

With a MultiIndex#

Unstacking when the columns are a MultiIndex is also careful about doing the right thing:

In [44]: df[:3].unstack(0)
Out[44]: 
exp            A                   B  ...                   A          
animal       cat                 dog  ...       cat       dog          
first        bar       baz       bar  ...       baz       bar       baz
second                                ...                              
one     0.895717  0.410835  0.805244  ...  0.132003  2.565646 -0.827317
two     1.431256       NaN  1.340309  ...       NaN -0.226169       NaN

[2 rows x 8 columns]

In [45]: df2.unstack(1)
Out[45]: 
exp            A                   B  ...                   A          
animal       cat                 dog  ...       cat       dog          
second       one       two       one  ...       two       one       two
first                                 ...                              
bar     0.895717  1.431256  0.805244  ... -1.170299  2.565646 -0.226169
baz     0.410835       NaN  0.813850  ...       NaN -0.827317       NaN
foo    -1.413681  0.875906  1.607920  ...  0.974466  0.569605 -2.006747
qux          NaN -1.226825       NaN  ... -1.281247       NaN -0.727707

[4 rows x 8 columns]

Reshaping by melt#

../_images/reshaping_melt.png

The top-level melt() function and the corresponding DataFrame.melt() are useful to massage a DataFrame into a format where one or more columns are identifier variables, while all other columns, considered measured variables, are “unpivoted” to the row axis, leaving just two non-identifier columns, “variable” and “value”. The names of those columns can be customized by supplying the var_name and value_name parameters.

For instance,

In [46]: cheese = pd.DataFrame(
   ....:     {
   ....:         "first": ["John", "Mary"],
   ....:         "last": ["Doe", "Bo"],
   ....:         "height": [5.5, 6.0],
   ....:         "weight": [130, 150],
   ....:     }
   ....: )
   ....: 

In [47]: cheese
Out[47]: 
  first last  height  weight
0  John  Doe     5.5     130
1  Mary   Bo     6.0     150

In [48]: cheese.melt(id_vars=["first", "last"])
Out[48]: 
  first last variable  value
0  John  Doe   height    5.5
1  Mary   Bo   height    6.0
2  John  Doe   weight  130.0
3  Mary   Bo   weight  150.0

In [49]: cheese.melt(id_vars=["first", "last"], var_name="quantity")
Out[49]: 
  first last quantity  value
0  John  Doe   height    5.5
1  Mary   Bo   height    6.0
2  John  Doe   weight  130.0
3  Mary   Bo   weight  150.0

When transforming a DataFrame using melt(), the index will be ignored. The original index values can be kept around by setting the ignore_index parameter to False (default is True). This will however duplicate them.

New in version 1.1.0.

In [50]: index = pd.MultiIndex.from_tuples([("person", "A"), ("person", "B")])

In [51]: cheese = pd.DataFrame(
   ....:     {
   ....:         "first": ["John", "Mary"],
   ....:         "last": ["Doe", "Bo"],
   ....:         "height": [5.5, 6.0],
   ....:         "weight": [130, 150],
   ....:     },
   ....:     index=index,
   ....: )
   ....: 

In [52]: cheese
Out[52]: 
         first last  height  weight
person A  John  Doe     5.5     130
       B  Mary   Bo     6.0     150

In [53]: cheese.melt(id_vars=["first", "last"])
Out[53]: 
  first last variable  value
0  John  Doe   height    5.5
1  Mary   Bo   height    6.0
2  John  Doe   weight  130.0
3  Mary   Bo   weight  150.0

In [54]: cheese.melt(id_vars=["first", "last"], ignore_index=False)
Out[54]: 
         first last variable  value
person A  John  Doe   height    5.5
       B  Mary   Bo   height    6.0
       A  John  Doe   weight  130.0
       B  Mary   Bo   weight  150.0

Another way to transform is to use the wide_to_long() panel data convenience function. It is less flexible than melt(), but more user-friendly.

In [55]: dft = pd.DataFrame(
   ....:     {
   ....:         "A1970": {0: "a", 1: "b", 2: "c"},
   ....:         "A1980": {0: "d", 1: "e", 2: "f"},
   ....:         "B1970": {0: 2.5, 1: 1.2, 2: 0.7},
   ....:         "B1980": {0: 3.2, 1: 1.3, 2: 0.1},
   ....:         "X": dict(zip(range(3), np.random.randn(3))),
   ....:     }
   ....: )
   ....: 

In [56]: dft["id"] = dft.index

In [57]: dft
Out[57]: 
  A1970 A1980  B1970  B1980         X  id
0     a     d    2.5    3.2 -0.121306   0
1     b     e    1.2    1.3 -0.097883   1
2     c     f    0.7    0.1  0.695775   2

In [58]: pd.wide_to_long(dft, ["A", "B"], i="id", j="year")
Out[58]: 
                X  A    B
id year                  
0  1970 -0.121306  a  2.5
1  1970 -0.097883  b  1.2
2  1970  0.695775  c  0.7
0  1980 -0.121306  d  3.2
1  1980 -0.097883  e  1.3
2  1980  0.695775  f  0.1

Combining with stats and GroupBy#

It should be no shock that combining pivot() / stack() / unstack() with GroupBy and the basic Series and DataFrame statistical functions can produce some very expressive and fast data manipulations.

In [59]: df
Out[59]: 
exp                  A         B                   A
animal             cat       dog       cat       dog
first second                                        
bar   one     0.895717  0.805244 -1.206412  2.565646
      two     1.431256  1.340309 -1.170299 -0.226169
baz   one     0.410835  0.813850  0.132003 -0.827317
      two    -0.076467 -1.187678  1.130127 -1.436737
foo   one    -1.413681  1.607920  1.024180  0.569605
      two     0.875906 -2.211372  0.974466 -2.006747
qux   one    -0.410001 -0.078638  0.545952 -1.219217
      two    -1.226825  0.769804 -1.281247 -0.727707

In [60]: df.stack().mean(1).unstack()
Out[60]: 
animal             cat       dog
first second                    
bar   one    -0.155347  1.685445
      two     0.130479  0.557070
baz   one     0.271419 -0.006733
      two     0.526830 -1.312207
foo   one    -0.194750  1.088763
      two     0.925186 -2.109060
qux   one     0.067976 -0.648927
      two    -1.254036  0.021048

# same result, another way
In [61]: df.groupby(level=1, axis=1).mean()
Out[61]: 
animal             cat       dog
first second                    
bar   one    -0.155347  1.685445
      two     0.130479  0.557070
baz   one     0.271419 -0.006733
      two     0.526830 -1.312207
foo   one    -0.194750  1.088763
      two     0.925186 -2.109060
qux   one     0.067976 -0.648927
      two    -1.254036  0.021048

In [62]: df.stack().groupby(level=1).mean()
Out[62]: 
exp            A         B
second                    
one     0.071448  0.455513
two    -0.424186 -0.204486

In [63]: df.mean().unstack(0)
Out[63]: 
exp            A         B
animal                    
cat     0.060843  0.018596
dog    -0.413580  0.232430

Pivot tables#

While pivot() provides general purpose pivoting with various data types (strings, numerics, etc.), pandas also provides pivot_table() for pivoting with aggregation of numeric data.

The function pivot_table() can be used to create spreadsheet-style pivot tables. See the cookbook for some advanced strategies.

It takes a number of arguments:

  • data: a DataFrame object.

  • values: a column or a list of columns to aggregate.

  • index: a column, Grouper, array which has the same length as data, or list of them. Keys to group by on the pivot table index. If an array is passed, it is being used as the same manner as column values.

  • columns: a column, Grouper, array which has the same length as data, or list of them. Keys to group by on the pivot table column. If an array is passed, it is being used as the same manner as column values.

  • aggfunc: function to use for aggregation, defaulting to numpy.mean.

Consider a data set like this:

In [64]: import datetime

In [65]: df = pd.DataFrame(
   ....:     {
   ....:         "A": ["one", "one", "two", "three"] * 6,
   ....:         "B": ["A", "B", "C"] * 8,
   ....:         "C": ["foo", "foo", "foo", "bar", "bar", "bar"] * 4,
   ....:         "D": np.random.randn(24),
   ....:         "E": np.random.randn(24),
   ....:         "F": [datetime.datetime(2013, i, 1) for i in range(1, 13)]
   ....:         + [datetime.datetime(2013, i, 15) for i in range(1, 13)],
   ....:     }
   ....: )
   ....: 

In [66]: df
Out[66]: 
        A  B    C         D         E          F
0     one  A  foo  0.341734 -0.317441 2013-01-01
1     one  B  foo  0.959726 -1.236269 2013-02-01
2     two  C  foo -1.110336  0.896171 2013-03-01
3   three  A  bar -0.619976 -0.487602 2013-04-01
4     one  B  bar  0.149748 -0.082240 2013-05-01
..    ... ..  ...       ...       ...        ...
19  three  B  foo  0.690579 -2.213588 2013-08-15
20    one  C  foo  0.995761  1.063327 2013-09-15
21    one  A  bar  2.396780  1.266143 2013-10-15
22    two  B  bar  0.014871  0.299368 2013-11-15
23  three  C  bar  3.357427 -0.863838 2013-12-15

[24 rows x 6 columns]

We can produce pivot tables from this data very easily:

In [67]: pd.pivot_table(df, values="D", index=["A", "B"], columns=["C"])
Out[67]: 
C             bar       foo
A     B                    
one   A  1.120915 -0.514058
      B -0.338421  0.002759
      C -0.538846  0.699535
three A -1.181568       NaN
      B       NaN  0.433512
      C  0.588783       NaN
two   A       NaN  1.000985
      B  0.158248       NaN
      C       NaN  0.176180

In [68]: pd.pivot_table(df, values="D", index=["B"], columns=["A", "C"], aggfunc=np.sum)
Out[68]: 
A       one               three                 two          
C       bar       foo       bar       foo       bar       foo
B                                                            
A  2.241830 -1.028115 -2.363137       NaN       NaN  2.001971
B -0.676843  0.005518       NaN  0.867024  0.316495       NaN
C -1.077692  1.399070  1.177566       NaN       NaN  0.352360

In [69]: pd.pivot_table(
   ....:     df, values=["D", "E"],
   ....:     index=["B"],
   ....:     columns=["A", "C"],
   ....:     aggfunc=np.sum,
   ....: )
   ....: 
Out[69]: 
          D                      ...         E                    
A       one               three  ...     three       two          
C       bar       foo       bar  ...       foo       bar       foo
B                                ...                              
A  2.241830 -1.028115 -2.363137  ...       NaN       NaN  0.128491
B -0.676843  0.005518       NaN  ... -2.128743 -0.194294       NaN
C -1.077692  1.399070  1.177566  ...       NaN       NaN  0.872482

[3 rows x 12 columns]

The result object is a DataFrame having potentially hierarchical indexes on the rows and columns. If the values column name is not given, the pivot table will include all of the data in an additional level of hierarchy in the columns:

In [70]: pd.pivot_table(df[["A", "B", "C", "D", "E"]], index=["A", "B"], columns=["C"])
Out[70]: 
                D                   E          
C             bar       foo       bar       foo
A     B                                        
one   A  1.120915 -0.514058  1.393057 -0.021605
      B -0.338421  0.002759  0.684140 -0.551692
      C -0.538846  0.699535 -0.988442  0.747859
three A -1.181568       NaN  0.961289       NaN
      B       NaN  0.433512       NaN -1.064372
      C  0.588783       NaN -0.131830       NaN
two   A       NaN  1.000985       NaN  0.064245
      B  0.158248       NaN -0.097147       NaN
      C       NaN  0.176180       NaN  0.436241

Also, you can use Grouper for index and columns keywords. For detail of Grouper, see Grouping with a Grouper specification.

In [71]: pd.pivot_table(df, values="D", index=pd.Grouper(freq="M", key="F"), columns="C")
Out[71]: 
C                bar       foo
F                             
2013-01-31       NaN -0.514058
2013-02-28       NaN  0.002759
2013-03-31       NaN  0.176180
2013-04-30 -1.181568       NaN
2013-05-31 -0.338421       NaN
2013-06-30 -0.538846       NaN
2013-07-31       NaN  1.000985
2013-08-31       NaN  0.433512
2013-09-30       NaN  0.699535
2013-10-31  1.120915       NaN
2013-11-30  0.158248       NaN
2013-12-31  0.588783       NaN

You can render a nice output of the table omitting the missing values by calling to_string() if you wish:

In [72]: table = pd.pivot_table(df, index=["A", "B"], columns=["C"], values=["D", "E"])

In [73]: print(table.to_string(na_rep=""))
                D                   E          
C             bar       foo       bar       foo
A     B                                        
one   A  1.120915 -0.514058  1.393057 -0.021605
      B -0.338421  0.002759  0.684140 -0.551692
      C -0.538846  0.699535 -0.988442  0.747859
three A -1.181568            0.961289          
      B            0.433512           -1.064372
      C  0.588783           -0.131830          
two   A            1.000985            0.064245
      B  0.158248           -0.097147          
      C            0.176180            0.436241
Note that pivot_table() is also available as an instance method on DataFrame,

i.e. DataFrame.pivot_table().

Adding margins#

If you pass margins=True to pivot_table(), special All columns and rows will be added with partial group aggregates across the categories on the rows and columns:

In [74]: table = df.pivot_table(
   ....:     index=["A", "B"],
   ....:     columns="C",
   ....:     values=["D", "E"],
   ....:     margins=True,
   ....:     aggfunc=np.std
   ....: )
   ....: 

In [75]: table
Out[75]: 
                D                             E                    
C             bar       foo       All       bar       foo       All
A     B                                                            
one   A  1.804346  1.210272  1.569879  0.179483  0.418374  0.858005
      B  0.690376  1.353355  0.898998  1.083825  0.968138  1.101401
      C  0.273641  0.418926  0.771139  1.689271  0.446140  1.422136
three A  0.794212       NaN  0.794212  2.049040       NaN  2.049040
      B       NaN  0.363548  0.363548       NaN  1.625237  1.625237
      C  3.915454       NaN  3.915454  1.035215       NaN  1.035215
two   A       NaN  0.442998  0.442998       NaN  0.447104  0.447104
      B  0.202765       NaN  0.202765  0.560757       NaN  0.560757
      C       NaN  1.819408  1.819408       NaN  0.650439  0.650439
All      1.556686  0.952552  1.246608  1.250924  0.899904  1.059389

Additionally, you can call DataFrame.stack() to display a pivoted DataFrame as having a multi-level index:

In [76]: table.stack()
Out[76]: 
                  D         E
A   B C                      
one A All  1.569879  0.858005
      bar  1.804346  0.179483
      foo  1.210272  0.418374
    B All  0.898998  1.101401
      bar  0.690376  1.083825
...             ...       ...
two C All  1.819408  0.650439
      foo  1.819408  0.650439
All   All  1.246608  1.059389
      bar  1.556686  1.250924
      foo  0.952552  0.899904

[24 rows x 2 columns]

Cross tabulations#

Use crosstab() to compute a cross-tabulation of two (or more) factors. By default crosstab() computes a frequency table of the factors unless an array of values and an aggregation function are passed.

It takes a number of arguments

  • index: array-like, values to group by in the rows.

  • columns: array-like, values to group by in the columns.

  • values: array-like, optional, array of values to aggregate according to the factors.

  • aggfunc: function, optional, If no values array is passed, computes a frequency table.

  • rownames: sequence, default None, must match number of row arrays passed.

  • colnames: sequence, default None, if passed, must match number of column arrays passed.

  • margins: boolean, default False, Add row/column margins (subtotals)

  • normalize: boolean, {‘all’, ‘index’, ‘columns’}, or {0,1}, default False. Normalize by dividing all values by the sum of values.

Any Series passed will have their name attributes used unless row or column names for the cross-tabulation are specified

For example:

In [77]: foo, bar, dull, shiny, one, two = "foo", "bar", "dull", "shiny", "one", "two"

In [78]: a = np.array([foo, foo, bar, bar, foo, foo], dtype=object)

In [79]: b = np.array([one, one, two, one, two, one], dtype=object)

In [80]: c = np.array([dull, dull, shiny, dull, dull, shiny], dtype=object)

In [81]: pd.crosstab(a, [b, c], rownames=["a"], colnames=["b", "c"])
Out[81]: 
b    one        two      
c   dull shiny dull shiny
a                        
bar    1     0    0     1
foo    2     1    1     0

If crosstab() receives only two Series, it will provide a frequency table.

In [82]: df = pd.DataFrame(
   ....:     {"A": [1, 2, 2, 2, 2], "B": [3, 3, 4, 4, 4], "C": [1, 1, np.nan, 1, 1]}
   ....: )
   ....: 

In [83]: df
Out[83]: 
   A  B    C
0  1  3  1.0
1  2  3  1.0
2  2  4  NaN
3  2  4  1.0
4  2  4  1.0

In [84]: pd.crosstab(df["A"], df["B"])
Out[84]: 
B  3  4
A      
1  1  0
2  1  3

crosstab() can also be implemented to Categorical data.

In [85]: foo = pd.Categorical(["a", "b"], categories=["a", "b", "c"])

In [86]: bar = pd.Categorical(["d", "e"], categories=["d", "e", "f"])

In [87]: pd.crosstab(foo, bar)
Out[87]: 
col_0  d  e
row_0      
a      1  0
b      0  1

If you want to include all of data categories even if the actual data does not contain any instances of a particular category, you should set dropna=False.

For example:

In [88]: pd.crosstab(foo, bar, dropna=False)
Out[88]: 
col_0  d  e  f
row_0         
a      1  0  0
b      0  1  0
c      0  0  0

Normalization#

Frequency tables can also be normalized to show percentages rather than counts using the normalize argument:

In [89]: pd.crosstab(df["A"], df["B"], normalize=True)
Out[89]: 
B    3    4
A          
1  0.2  0.0
2  0.2  0.6

normalize can also normalize values within each row or within each column:

In [90]: pd.crosstab(df["A"], df["B"], normalize="columns")
Out[90]: 
B    3    4
A          
1  0.5  0.0
2  0.5  1.0

crosstab() can also be passed a third Series and an aggregation function (aggfunc) that will be applied to the values of the third Series within each group defined by the first two Series:

In [91]: pd.crosstab(df["A"], df["B"], values=df["C"], aggfunc=np.sum)
Out[91]: 
B    3    4
A          
1  1.0  NaN
2  1.0  2.0

Adding margins#

Finally, one can also add margins or normalize this output.

In [92]: pd.crosstab(
   ....:     df["A"], df["B"], values=df["C"], aggfunc=np.sum, normalize=True, margins=True
   ....: )
   ....: 
Out[92]: 
B       3    4   All
A                   
1    0.25  0.0  0.25
2    0.25  0.5  0.75
All  0.50  0.5  1.00

Tiling#

The cut() function computes groupings for the values of the input array and is often used to transform continuous variables to discrete or categorical variables:

In [93]: ages = np.array([10, 15, 13, 12, 23, 25, 28, 59, 60])

In [94]: pd.cut(ages, bins=3)
Out[94]: 
[(9.95, 26.667], (9.95, 26.667], (9.95, 26.667], (9.95, 26.667], (9.95, 26.667], (9.95, 26.667], (26.667, 43.333], (43.333, 60.0], (43.333, 60.0]]
Categories (3, interval[float64, right]): [(9.95, 26.667] < (26.667, 43.333] < (43.333, 60.0]]

If the bins keyword is an integer, then equal-width bins are formed. Alternatively we can specify custom bin-edges:

In [95]: c = pd.cut(ages, bins=[0, 18, 35, 70])

In [96]: c
Out[96]: 
[(0, 18], (0, 18], (0, 18], (0, 18], (18, 35], (18, 35], (18, 35], (35, 70], (35, 70]]
Categories (3, interval[int64, right]): [(0, 18] < (18, 35] < (35, 70]]

If the bins keyword is an IntervalIndex, then these will be used to bin the passed data.:

pd.cut([25, 20, 50], bins=c.categories)

Computing indicator / dummy variables#

To convert a categorical variable into a “dummy” or “indicator” DataFrame, for example a column in a DataFrame (a Series) which has k distinct values, can derive a DataFrame containing k columns of 1s and 0s using get_dummies():

In [97]: df = pd.DataFrame({"key": list("bbacab"), "data1": range(6)})

In [98]: pd.get_dummies(df["key"])
Out[98]: 
       a      b      c
0  False   True  False
1  False   True  False
2   True  False  False
3  False  False   True
4   True  False  False
5  False   True  False

Sometimes it’s useful to prefix the column names, for example when merging the result with the original DataFrame:

In [99]: dummies = pd.get_dummies(df["key"], prefix="key")

In [100]: dummies
Out[100]: 
   key_a  key_b  key_c
0  False   True  False
1  False   True  False
2   True  False  False
3  False  False   True
4   True  False  False
5  False   True  False

In [101]: df[["data1"]].join(dummies)
Out[101]: 
   data1  key_a  key_b  key_c
0      0  False   True  False
1      1  False   True  False
2      2   True  False  False
3      3  False  False   True
4      4   True  False  False
5      5  False   True  False

This function is often used along with discretization functions like cut():

In [102]: values = np.random.randn(10)

In [103]: values
Out[103]: 
array([ 0.4082, -1.0481, -0.0257, -0.9884,  0.0941,  1.2627,  1.29  ,
        0.0824, -0.0558,  0.5366])

In [104]: bins = [0, 0.2, 0.4, 0.6, 0.8, 1]

In [105]: pd.get_dummies(pd.cut(values, bins))
Out[105]: 
   (0.0, 0.2]  (0.2, 0.4]  (0.4, 0.6]  (0.6, 0.8]  (0.8, 1.0]
0       False       False        True       False       False
1       False       False       False       False       False
2       False       False       False       False       False
3       False       False       False       False       False
4        True       False       False       False       False
5       False       False       False       False       False
6       False       False       False       False       False
7        True       False       False       False       False
8       False       False       False       False       False
9       False       False        True       False       False

See also Series.str.get_dummies.

get_dummies() also accepts a DataFrame. By default all categorical variables (categorical in the statistical sense, those with object or categorical dtype) are encoded as dummy variables.

In [106]: df = pd.DataFrame({"A": ["a", "b", "a"], "B": ["c", "c", "b"], "C": [1, 2, 3]})

In [107]: pd.get_dummies(df)
Out[107]: 
   C    A_a    A_b    B_b    B_c
0  1   True  False  False   True
1  2  False   True  False   True
2  3   True  False   True  False

All non-object columns are included untouched in the output. You can control the columns that are encoded with the columns keyword.

In [108]: pd.get_dummies(df, columns=["A"])
Out[108]: 
   B  C    A_a    A_b
0  c  1   True  False
1  c  2  False   True
2  b  3   True  False

Notice that the B column is still included in the output, it just hasn’t been encoded. You can drop B before calling get_dummies if you don’t want to include it in the output.

As with the Series version, you can pass values for the prefix and prefix_sep. By default the column name is used as the prefix, and _ as the prefix separator. You can specify prefix and prefix_sep in 3 ways:

  • string: Use the same value for prefix or prefix_sep for each column to be encoded.

  • list: Must be the same length as the number of columns being encoded.

  • dict: Mapping column name to prefix.

In [109]: simple = pd.get_dummies(df, prefix="new_prefix")

In [110]: simple
Out[110]: 
   C  new_prefix_a  new_prefix_b  new_prefix_b  new_prefix_c
0  1          True         False         False          True
1  2         False          True         False          True
2  3          True         False          True         False

In [111]: from_list = pd.get_dummies(df, prefix=["from_A", "from_B"])

In [112]: from_list
Out[112]: 
   C  from_A_a  from_A_b  from_B_b  from_B_c
0  1      True     False     False      True
1  2     False      True     False      True
2  3      True     False      True     False

In [113]: from_dict = pd.get_dummies(df, prefix={"B": "from_B", "A": "from_A"})

In [114]: from_dict
Out[114]: 
   C  from_A_a  from_A_b  from_B_b  from_B_c
0  1      True     False     False      True
1  2     False      True     False      True
2  3      True     False      True     False

Sometimes it will be useful to only keep k-1 levels of a categorical variable to avoid collinearity when feeding the result to statistical models. You can switch to this mode by turn on drop_first.

In [115]: s = pd.Series(list("abcaa"))

In [116]: pd.get_dummies(s)
Out[116]: 
       a      b      c
0   True  False  False
1  False   True  False
2  False  False   True
3   True  False  False
4   True  False  False

In [117]: pd.get_dummies(s, drop_first=True)
Out[117]: 
       b      c
0  False  False
1   True  False
2  False   True
3  False  False
4  False  False

When a column contains only one level, it will be omitted in the result.

In [118]: df = pd.DataFrame({"A": list("aaaaa"), "B": list("ababc")})

In [119]: pd.get_dummies(df)
Out[119]: 
    A_a    B_a    B_b    B_c
0  True   True  False  False
1  True  False   True  False
2  True   True  False  False
3  True  False   True  False
4  True  False  False   True

In [120]: pd.get_dummies(df, drop_first=True)
Out[120]: 
     B_b    B_c
0  False  False
1   True  False
2  False  False
3   True  False
4  False   True

By default new columns will have np.uint8 dtype. To choose another dtype, use the dtype argument:

In [121]: df = pd.DataFrame({"A": list("abc"), "B": [1.1, 2.2, 3.3]})

In [122]: pd.get_dummies(df, dtype=bool).dtypes
Out[122]: 
B      float64
A_a       bool
A_b       bool
A_c       bool
dtype: object

New in version 1.5.0.

To convert a “dummy” or “indicator” DataFrame, into a categorical DataFrame, for example k columns of a DataFrame containing 1s and 0s can derive a DataFrame which has k distinct values using from_dummies():

In [123]: df = pd.DataFrame({"prefix_a": [0, 1, 0], "prefix_b": [1, 0, 1]})

In [124]: df
Out[124]: 
   prefix_a  prefix_b
0         0         1
1         1         0
2         0         1

In [125]: pd.from_dummies(df, sep="_")
Out[125]: 
  prefix
0      b
1      a
2      b

Dummy coded data only requires k - 1 categories to be included, in this case the k th category is the default category, implied by not being assigned any of the other k - 1 categories, can be passed via default_category.

In [126]: df = pd.DataFrame({"prefix_a": [0, 1, 0]})

In [127]: df
Out[127]: 
   prefix_a
0         0
1         1
2         0

In [128]: pd.from_dummies(df, sep="_", default_category="b")
Out[128]: 
  prefix
0      b
1      a
2      b

Factorizing values#

To encode 1-d values as an enumerated type use factorize():

In [129]: x = pd.Series(["A", "A", np.nan, "B", 3.14, np.inf])

In [130]: x
Out[130]: 
0       A
1       A
2     NaN
3       B
4    3.14
5     inf
dtype: object

In [131]: labels, uniques = pd.factorize(x)

In [132]: labels
Out[132]: array([ 0,  0, -1,  1,  2,  3])

In [133]: uniques
Out[133]: Index(['A', 'B', 3.14, inf], dtype='object')

Note that factorize() is similar to numpy.unique, but differs in its handling of NaN:

Note

The following numpy.unique will fail under Python 3 with a TypeError because of an ordering bug. See also here.

In [134]: ser = pd.Series(['A', 'A', np.nan, 'B', 3.14, np.inf])

In [135]: pd.factorize(ser, sort=True)
Out[135]: (array([ 2,  2, -1,  3,  0,  1]), Index([3.14, inf, 'A', 'B'], dtype='object'))

In [136]: np.unique(ser, return_inverse=True)[::-1]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[136], line 1
----> 1 np.unique(ser, return_inverse=True)[::-1]

File <__array_function__ internals>:200, in unique(*args, **kwargs)

File ~/micromamba-root/envs/test/lib/python3.8/site-packages/numpy/lib/arraysetops.py:274, in unique(ar, return_index, return_inverse, return_counts, axis, equal_nan)
    272 ar = np.asanyarray(ar)
    273 if axis is None:
--> 274     ret = _unique1d(ar, return_index, return_inverse, return_counts, 
    275                     equal_nan=equal_nan)
    276     return _unpack_tuple(ret)
    278 # axis was specified and not None

File ~/micromamba-root/envs/test/lib/python3.8/site-packages/numpy/lib/arraysetops.py:333, in _unique1d(ar, return_index, return_inverse, return_counts, equal_nan)
    330 optional_indices = return_index or return_inverse
    332 if optional_indices:
--> 333     perm = ar.argsort(kind='mergesort' if return_index else 'quicksort')
    334     aux = ar[perm]
    335 else:

TypeError: '<' not supported between instances of 'float' and 'str'

Note

If you just want to handle one column as a categorical variable (like R’s factor), you can use df["cat_col"] = pd.Categorical(df["col"]) or df["cat_col"] = df["col"].astype("category"). For full docs on Categorical, see the Categorical introduction and the API documentation.

Examples#

In this section, we will review frequently asked questions and examples. The column names and relevant column values are named to correspond with how this DataFrame will be pivoted in the answers below.

In [137]: np.random.seed([3, 1415])

In [138]: n = 20

In [139]: cols = np.array(["key", "row", "item", "col"])

In [140]: df = cols + pd.DataFrame(
   .....:     (np.random.randint(5, size=(n, 4)) // [2, 1, 2, 1]).astype(str)
   .....: )
   .....: 

In [141]: df.columns = cols

In [142]: df = df.join(pd.DataFrame(np.random.rand(n, 2).round(2)).add_prefix("val"))

In [143]: df
Out[143]: 
     key   row   item   col  val0  val1
0   key0  row3  item1  col3  0.81  0.04
1   key1  row2  item1  col2  0.44  0.07
2   key1  row0  item1  col0  0.77  0.01
3   key0  row4  item0  col2  0.15  0.59
4   key1  row0  item2  col1  0.81  0.64
..   ...   ...    ...   ...   ...   ...
15  key0  row3  item1  col1  0.31  0.23
16  key0  row0  item2  col3  0.86  0.01
17  key0  row4  item0  col3  0.64  0.21
18  key2  row2  item2  col0  0.13  0.45
19  key0  row2  item0  col4  0.37  0.70

[20 rows x 6 columns]

Pivoting with single aggregations#

Suppose we wanted to pivot df such that the col values are columns, row values are the index, and the mean of val0 are the values? In particular, the resulting DataFrame should look like:

col   col0   col1   col2   col3  col4
row
row0  0.77  0.605    NaN  0.860  0.65
row2  0.13    NaN  0.395  0.500  0.25
row3   NaN  0.310    NaN  0.545   NaN
row4   NaN  0.100  0.395  0.760  0.24

This solution uses pivot_table(). Also note that aggfunc='mean' is the default. It is included here to be explicit.

In [144]: df.pivot_table(values="val0", index="row", columns="col", aggfunc="mean")
Out[144]: 
col   col0   col1   col2   col3  col4
row                                  
row0  0.77  0.605    NaN  0.860  0.65
row2  0.13    NaN  0.395  0.500  0.25
row3   NaN  0.310    NaN  0.545   NaN
row4   NaN  0.100  0.395  0.760  0.24

Note that we can also replace the missing values by using the fill_value parameter.

In [145]: df.pivot_table(
   .....:     values="val0",
   .....:     index="row",
   .....:     columns="col",
   .....:     aggfunc="mean",
   .....:     fill_value=0,
   .....: )
   .....: 
Out[145]: 
col   col0   col1   col2   col3  col4
row                                  
row0  0.77  0.605  0.000  0.860  0.65
row2  0.13  0.000  0.395  0.500  0.25
row3  0.00  0.310  0.000  0.545  0.00
row4  0.00  0.100  0.395  0.760  0.24

Also note that we can pass in other aggregation functions as well. For example, we can also pass in sum.

In [146]: df.pivot_table(
   .....:     values="val0",
   .....:     index="row",
   .....:     columns="col",
   .....:     aggfunc="sum",
   .....:     fill_value=0,
   .....: )
   .....: 
Out[146]: 
col   col0  col1  col2  col3  col4
row                               
row0  0.77  1.21  0.00  0.86  0.65
row2  0.13  0.00  0.79  0.50  0.50
row3  0.00  0.31  0.00  1.09  0.00
row4  0.00  0.10  0.79  1.52  0.24

Another aggregation we can do is calculate the frequency in which the columns and rows occur together a.k.a. “cross tabulation”. To do this, we can pass size to the aggfunc parameter.

In [147]: df.pivot_table(index="row", columns="col", fill_value=0, aggfunc="size")
Out[147]: 
col   col0  col1  col2  col3  col4
row                               
row0     1     2     0     1     1
row2     1     0     2     1     2
row3     0     1     0     2     0
row4     0     1     2     2     1

Pivoting with multiple aggregations#

We can also perform multiple aggregations. For example, to perform both a sum and mean, we can pass in a list to the aggfunc argument.

In [148]: df.pivot_table(
   .....:     values="val0",
   .....:     index="row",
   .....:     columns="col",
   .....:     aggfunc=["mean", "sum"],
   .....: )
   .....: 
Out[148]: 
      mean                              sum                        
col   col0   col1   col2   col3  col4  col0  col1  col2  col3  col4
row                                                                
row0  0.77  0.605    NaN  0.860  0.65  0.77  1.21   NaN  0.86  0.65
row2  0.13    NaN  0.395  0.500  0.25  0.13   NaN  0.79  0.50  0.50
row3   NaN  0.310    NaN  0.545   NaN   NaN  0.31   NaN  1.09   NaN
row4   NaN  0.100  0.395  0.760  0.24   NaN  0.10  0.79  1.52  0.24

Note to aggregate over multiple value columns, we can pass in a list to the values parameter.

In [149]: df.pivot_table(
   .....:     values=["val0", "val1"],
   .....:     index="row",
   .....:     columns="col",
   .....:     aggfunc=["mean"],
   .....: )
   .....: 
Out[149]: 
      mean                                                           
      val0                             val1                          
col   col0   col1   col2   col3  col4  col0   col1  col2   col3  col4
row                                                                  
row0  0.77  0.605    NaN  0.860  0.65  0.01  0.745   NaN  0.010  0.02
row2  0.13    NaN  0.395  0.500  0.25  0.45    NaN  0.34  0.440  0.79
row3   NaN  0.310    NaN  0.545   NaN   NaN  0.230   NaN  0.075   NaN
row4   NaN  0.100  0.395  0.760  0.24   NaN  0.070  0.42  0.300  0.46

Note to subdivide over multiple columns we can pass in a list to the columns parameter.

In [150]: df.pivot_table(
   .....:     values=["val0"],
   .....:     index="row",
   .....:     columns=["item", "col"],
   .....:     aggfunc=["mean"],
   .....: )
   .....: 
Out[150]: 
      mean                                                                   
      val0                                                                   
item item0             item1                         item2                   
col   col2  col3  col4  col0  col1  col2  col3  col4  col0   col1  col3  col4
row                                                                          
row0   NaN   NaN   NaN  0.77   NaN   NaN   NaN   NaN   NaN  0.605  0.86  0.65
row2  0.35   NaN  0.37   NaN   NaN  0.44   NaN   NaN  0.13    NaN  0.50  0.13
row3   NaN   NaN   NaN   NaN  0.31   NaN  0.81   NaN   NaN    NaN  0.28   NaN
row4  0.15  0.64   NaN   NaN  0.10  0.64  0.88  0.24   NaN    NaN   NaN   NaN

Exploding a list-like column#

Sometimes the values in a column are list-like.

In [151]: keys = ["panda1", "panda2", "panda3"]

In [152]: values = [["eats", "shoots"], ["shoots", "leaves"], ["eats", "leaves"]]

In [153]: df = pd.DataFrame({"keys": keys, "values": values})

In [154]: df
Out[154]: 
     keys            values
0  panda1    [eats, shoots]
1  panda2  [shoots, leaves]
2  panda3    [eats, leaves]

We can ‘explode’ the values column, transforming each list-like to a separate row, by using explode(). This will replicate the index values from the original row:

In [155]: df["values"].explode()
Out[155]: 
0      eats
0    shoots
1    shoots
1    leaves
2      eats
2    leaves
Name: values, dtype: object

You can also explode the column in the DataFrame.

In [156]: df.explode("values")
Out[156]: 
     keys  values
0  panda1    eats
0  panda1  shoots
1  panda2  shoots
1  panda2  leaves
2  panda3    eats
2  panda3  leaves

Series.explode() will replace empty lists with np.nan and preserve scalar entries. The dtype of the resulting Series is always object.

In [157]: s = pd.Series([[1, 2, 3], "foo", [], ["a", "b"]])

In [158]: s
Out[158]: 
0    [1, 2, 3]
1          foo
2           []
3       [a, b]
dtype: object

In [159]: s.explode()
Out[159]: 
0      1
0      2
0      3
1    foo
2    NaN
3      a
3      b
dtype: object

Here is a typical usecase. You have comma separated strings in a column and want to expand this.

In [160]: df = pd.DataFrame([{"var1": "a,b,c", "var2": 1}, {"var1": "d,e,f", "var2": 2}])

In [161]: df
Out[161]: 
    var1  var2
0  a,b,c     1
1  d,e,f     2

Creating a long form DataFrame is now straightforward using explode and chained operations

In [162]: df.assign(var1=df.var1.str.split(",")).explode("var1")
Out[162]: 
  var1  var2
0    a     1
0    b     1
0    c     1
1    d     2
1    e     2
1    f     2