Data is often stored in so-called “stacked” or “record” format:
In [1]: df Out[1]: 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
For the curious here is how the above DataFrame was created:
DataFrame
import pandas._testing as tm 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"]) df = unpivot(tm.makeTimeDataFrame(3))
To select out everything for variable A we could do:
A
In [2]: df[df["variable"] == "A"] Out[2]: 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()):
columns
index
DataFrame.pivot()
pivot()
In [3]: df.pivot(index="date", columns="variable", values="value") Out[3]: 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:
values
pivot
In [4]: df["value2"] = df["value"] * 2 In [5]: pivoted = df.pivot(index="date", columns="variable") In [6]: pivoted Out[6]: value value2 variable A B C D A B C D date 2000-01-03 0.469112 -1.135632 0.119209 -2.104569 0.938225 -2.271265 0.238417 -4.209138 2000-01-04 -0.282863 1.212112 -1.044236 -0.494929 -0.565727 2.424224 -2.088472 -0.989859 2000-01-05 -1.509059 -0.173215 -0.861849 1.071804 -3.018117 -0.346429 -1.723698 2.143608
You can then select subsets from the pivoted DataFrame:
In [7]: pivoted["value2"] Out[7]: 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.
ValueError: Index contains duplicate entries, cannot reshape
pivot_table()
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()
unstack()
Series
MultiIndex
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.
stack
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.
unstack
The clearest way to explain is by example. Let’s take a prior example data set from the hierarchical indexing section:
In [8]: tuples = list( ...: zip( ...: *[ ...: ["bar", "bar", "baz", "baz", "foo", "foo", "qux", "qux"], ...: ["one", "two", "one", "two", "one", "two", "one", "two"], ...: ] ...: ) ...: ) ...: In [9]: index = pd.MultiIndex.from_tuples(tuples, names=["first", "second"]) In [10]: df = pd.DataFrame(np.random.randn(8, 2), index=index, columns=["A", "B"]) In [11]: df2 = df[:4] In [12]: df2 Out[12]: 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’s columns to produce either:
A Series, in the case of a simple column Index.
A DataFrame, in the case of a MultiIndex in the columns.
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 [13]: stacked = df2.stack() In [14]: stacked Out[14]: 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 [15]: stacked.unstack() Out[15]: 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 [16]: stacked.unstack(1) Out[16]: 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 [17]: stacked.unstack(0) Out[17]: 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
If the indexes have names, you can use the level names instead of specifying the level numbers:
In [18]: stacked.unstack("second") Out[18]: 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
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 [19]: index = pd.MultiIndex.from_product([[2, 1], ["a", "b"]]) In [20]: df = pd.DataFrame(np.random.randn(4), index=index, columns=["A"]) In [21]: df Out[21]: A 2 a -0.370647 b -1.157892 1 a -1.344312 b 0.844885 In [22]: all(df.unstack().stack() == df.sort_index()) Out[22]: True
The above code will raise a TypeError if the call to sort_index is removed.
TypeError
sort_index
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 [23]: columns = pd.MultiIndex.from_tuples( ....: [ ....: ("A", "cat", "long"), ....: ("B", "cat", "long"), ....: ("A", "dog", "short"), ....: ("B", "dog", "short"), ....: ], ....: names=["exp", "animal", "hair_length"], ....: ) ....: In [24]: df = pd.DataFrame(np.random.randn(4, 4), columns=columns) In [25]: df Out[25]: 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 [26]: df.stack(level=["animal", "hair_length"]) Out[26]: 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 [27]: df.stack(level=[1, 2]) Out[27]: 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
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 [28]: columns = pd.MultiIndex.from_tuples( ....: [ ....: ("A", "cat"), ....: ("B", "dog"), ....: ("B", "cat"), ....: ("A", "dog"), ....: ], ....: names=["exp", "animal"], ....: ) ....: In [29]: index = pd.MultiIndex.from_product( ....: [("bar", "baz", "foo", "qux"), ("one", "two")], names=["first", "second"] ....: ) ....: In [30]: df = pd.DataFrame(np.random.randn(8, 4), index=index, columns=columns) In [31]: df2 = df.iloc[[0, 1, 2, 4, 5, 7]] In [32]: df2 Out[32]: 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:
level
In [33]: df2.stack("exp") Out[33]: 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 [34]: df2.stack("animal") Out[34]: 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.
NaN
NaT
In [35]: df3 = df.iloc[[0, 1, 4, 7], [1, 2]] In [36]: df3 Out[36]: 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 [37]: df3.unstack() Out[37]: 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.
fill_value
In [38]: df3.unstack(fill_value=-1e9) Out[38]: 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
Unstacking when the columns are a MultiIndex is also careful about doing the right thing:
In [39]: df[:3].unstack(0) Out[39]: exp A B A animal cat dog cat dog first bar baz bar baz bar baz bar baz second one 0.895717 0.410835 0.805244 0.81385 -1.206412 0.132003 2.565646 -0.827317 two 1.431256 NaN 1.340309 NaN -1.170299 NaN -0.226169 NaN In [40]: df2.unstack(1) Out[40]: exp A B A animal cat dog cat dog second one two one two one two one two first bar 0.895717 1.431256 0.805244 1.340309 -1.206412 -1.170299 2.565646 -0.226169 baz 0.410835 NaN 0.813850 NaN 0.132003 NaN -0.827317 NaN foo -1.413681 0.875906 1.607920 -2.211372 1.024180 0.974466 0.569605 -2.006747 qux NaN -1.226825 NaN 0.769804 NaN -1.281247 NaN -0.727707
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.
melt()
DataFrame.melt()
var_name
value_name
For instance,
In [41]: cheese = pd.DataFrame( ....: { ....: "first": ["John", "Mary"], ....: "last": ["Doe", "Bo"], ....: "height": [5.5, 6.0], ....: "weight": [130, 150], ....: } ....: ) ....: In [42]: cheese Out[42]: first last height weight 0 John Doe 5.5 130 1 Mary Bo 6.0 150 In [43]: cheese.melt(id_vars=["first", "last"]) Out[43]: 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 [44]: cheese.melt(id_vars=["first", "last"], var_name="quantity") Out[44]: 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.
ignore_index
False
True
New in version 1.1.0.
In [45]: index = pd.MultiIndex.from_tuples([("person", "A"), ("person", "B")]) In [46]: cheese = pd.DataFrame( ....: { ....: "first": ["John", "Mary"], ....: "last": ["Doe", "Bo"], ....: "height": [5.5, 6.0], ....: "weight": [130, 150], ....: }, ....: index=index, ....: ) ....: In [47]: cheese Out[47]: first last height weight person A John Doe 5.5 130 B 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"], ignore_index=False) Out[49]: 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.
wide_to_long()
In [50]: 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 [51]: dft["id"] = dft.index In [52]: dft Out[52]: 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 [53]: pd.wide_to_long(dft, ["A", "B"], i="id", j="year") Out[53]: 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
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 [54]: df Out[54]: 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 [55]: df.stack().mean(1).unstack() Out[55]: 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 [56]: df.groupby(level=1, axis=1).mean() Out[56]: 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 [57]: df.stack().groupby(level=1).mean() Out[57]: exp A B second one 0.071448 0.455513 two -0.424186 -0.204486 In [58]: df.mean().unstack(0) Out[58]: exp A B animal cat 0.060843 0.018596 dog -0.413580 0.232430
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.
data
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.
aggfunc
numpy.mean
Consider a data set like this:
In [59]: import datetime In [60]: 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 [61]: df Out[61]: 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 [62]: pd.pivot_table(df, values="D", index=["A", "B"], columns=["C"]) Out[62]: 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 [63]: pd.pivot_table(df, values="D", index=["B"], columns=["A", "C"], aggfunc=np.sum) Out[63]: 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 [64]: pd.pivot_table( ....: df, values=["D", "E"], ....: index=["B"], ....: columns=["A", "C"], ....: aggfunc=np.sum, ....: ) ....: Out[64]: D E A one three two one three two C bar foo bar foo bar foo bar foo bar foo bar foo B A 2.241830 -1.028115 -2.363137 NaN NaN 2.001971 2.786113 -0.043211 1.922577 NaN NaN 0.128491 B -0.676843 0.005518 NaN 0.867024 0.316495 NaN 1.368280 -1.103384 NaN -2.128743 -0.194294 NaN C -1.077692 1.399070 1.177566 NaN NaN 0.352360 -1.976883 1.495717 -0.263660 NaN NaN 0.872482
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 that can be aggregated in an additional level of hierarchy in the columns:
In [65]: pd.pivot_table(df, index=["A", "B"], columns=["C"]) Out[65]: 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.
Grouper
In [66]: pd.pivot_table(df, values="D", index=pd.Grouper(freq="M", key="F"), columns="C") Out[66]: 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:
to_string
In [67]: table = pd.pivot_table(df, index=["A", "B"], columns=["C"]) In [68]: 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
pivot_table
i.e. DataFrame.pivot_table().
DataFrame.pivot_table()
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:
margins=True
All
In [69]: df.pivot_table(index=["A", "B"], columns="C", margins=True, aggfunc=np.std) Out[69]: 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
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.
crosstab()
crosstab
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.
rownames
None
colnames: sequence, default None, if passed, must match number of column arrays passed.
colnames
margins: boolean, default False, Add row/column margins (subtotals)
margins
normalize: boolean, {‘all’, ‘index’, ‘columns’}, or {0,1}, default False. Normalize by dividing all values by the sum of values.
normalize
Any Series passed will have their name attributes used unless row or column names for the cross-tabulation are specified
For example:
In [70]: foo, bar, dull, shiny, one, two = "foo", "bar", "dull", "shiny", "one", "two" In [71]: a = np.array([foo, foo, bar, bar, foo, foo], dtype=object) In [72]: b = np.array([one, one, two, one, two, one], dtype=object) In [73]: c = np.array([dull, dull, shiny, dull, dull, shiny], dtype=object) In [74]: pd.crosstab(a, [b, c], rownames=["a"], colnames=["b", "c"]) Out[74]: 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 [75]: df = pd.DataFrame( ....: {"A": [1, 2, 2, 2, 2], "B": [3, 3, 4, 4, 4], "C": [1, 1, np.nan, 1, 1]} ....: ) ....: In [76]: df Out[76]: 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 [77]: pd.crosstab(df["A"], df["B"]) Out[77]: B 3 4 A 1 1 0 2 1 3
crosstab can also be implemented to Categorical data.
Categorical
In [78]: foo = pd.Categorical(["a", "b"], categories=["a", "b", "c"]) In [79]: bar = pd.Categorical(["d", "e"], categories=["d", "e", "f"]) In [80]: pd.crosstab(foo, bar) Out[80]: 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.
dropna=False
In [81]: pd.crosstab(foo, bar, dropna=False) Out[81]: col_0 d e f row_0 a 1 0 0 b 0 1 0 c 0 0 0
Frequency tables can also be normalized to show percentages rather than counts using the normalize argument:
In [82]: pd.crosstab(df["A"], df["B"], normalize=True) Out[82]: 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 [83]: pd.crosstab(df["A"], df["B"], normalize="columns") Out[83]: 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 [84]: pd.crosstab(df["A"], df["B"], values=df["C"], aggfunc=np.sum) Out[84]: B 3 4 A 1 1.0 NaN 2 1.0 2.0
Finally, one can also add margins or normalize this output.
In [85]: pd.crosstab( ....: df["A"], df["B"], values=df["C"], aggfunc=np.sum, normalize=True, margins=True ....: ) ....: Out[85]: 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
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:
cut()
In [86]: ages = np.array([10, 15, 13, 12, 23, 25, 28, 59, 60]) In [87]: pd.cut(ages, bins=3) Out[87]: [(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]): [(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:
bins
In [88]: c = pd.cut(ages, bins=[0, 18, 35, 70]) In [89]: c Out[89]: [(0, 18], (0, 18], (0, 18], (0, 18], (18, 35], (18, 35], (18, 35], (35, 70], (35, 70]] Categories (3, interval[int64]): [(0, 18] < (18, 35] < (35, 70]]
If the bins keyword is an IntervalIndex, then these will be used to bin the passed data.:
IntervalIndex
pd.cut([25, 20, 50], bins=c.categories)
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():
k
get_dummies()
In [90]: df = pd.DataFrame({"key": list("bbacab"), "data1": range(6)}) In [91]: pd.get_dummies(df["key"]) Out[91]: a b c 0 0 1 0 1 0 1 0 2 1 0 0 3 0 0 1 4 1 0 0 5 0 1 0
Sometimes it’s useful to prefix the column names, for example when merging the result with the original DataFrame:
In [92]: dummies = pd.get_dummies(df["key"], prefix="key") In [93]: dummies Out[93]: key_a key_b key_c 0 0 1 0 1 0 1 0 2 1 0 0 3 0 0 1 4 1 0 0 5 0 1 0 In [94]: df[["data1"]].join(dummies) Out[94]: data1 key_a key_b key_c 0 0 0 1 0 1 1 0 1 0 2 2 1 0 0 3 3 0 0 1 4 4 1 0 0 5 5 0 1 0
This function is often used along with discretization functions like cut:
cut
In [95]: values = np.random.randn(10) In [96]: values Out[96]: array([ 0.4082, -1.0481, -0.0257, -0.9884, 0.0941, 1.2627, 1.29 , 0.0824, -0.0558, 0.5366]) In [97]: bins = [0, 0.2, 0.4, 0.6, 0.8, 1] In [98]: pd.get_dummies(pd.cut(values, bins)) Out[98]: (0.0, 0.2] (0.2, 0.4] (0.4, 0.6] (0.6, 0.8] (0.8, 1.0] 0 0 0 1 0 0 1 0 0 0 0 0 2 0 0 0 0 0 3 0 0 0 0 0 4 1 0 0 0 0 5 0 0 0 0 0 6 0 0 0 0 0 7 1 0 0 0 0 8 0 0 0 0 0 9 0 0 1 0 0
See also Series.str.get_dummies.
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.
object
categorical
In [99]: df = pd.DataFrame({"A": ["a", "b", "a"], "B": ["c", "c", "b"], "C": [1, 2, 3]}) In [100]: pd.get_dummies(df) Out[100]: C A_a A_b B_b B_c 0 1 1 0 0 1 1 2 0 1 0 1 2 3 1 0 1 0
All non-object columns are included untouched in the output. You can control the columns that are encoded with the columns keyword.
In [101]: pd.get_dummies(df, columns=["A"]) Out[101]: B C A_a A_b 0 c 1 1 0 1 c 2 0 1 2 b 3 1 0
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.
B
get_dummies
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:
prefix
prefix_sep
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 [102]: simple = pd.get_dummies(df, prefix="new_prefix") In [103]: simple Out[103]: C new_prefix_a new_prefix_b new_prefix_b new_prefix_c 0 1 1 0 0 1 1 2 0 1 0 1 2 3 1 0 1 0 In [104]: from_list = pd.get_dummies(df, prefix=["from_A", "from_B"]) In [105]: from_list Out[105]: C from_A_a from_A_b from_B_b from_B_c 0 1 1 0 0 1 1 2 0 1 0 1 2 3 1 0 1 0 In [106]: from_dict = pd.get_dummies(df, prefix={"B": "from_B", "A": "from_A"}) In [107]: from_dict Out[107]: C from_A_a from_A_b from_B_b from_B_c 0 1 1 0 0 1 1 2 0 1 0 1 2 3 1 0 1 0
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.
drop_first
In [108]: s = pd.Series(list("abcaa")) In [109]: pd.get_dummies(s) Out[109]: a b c 0 1 0 0 1 0 1 0 2 0 0 1 3 1 0 0 4 1 0 0 In [110]: pd.get_dummies(s, drop_first=True) Out[110]: b c 0 0 0 1 1 0 2 0 1 3 0 0 4 0 0
When a column contains only one level, it will be omitted in the result.
In [111]: df = pd.DataFrame({"A": list("aaaaa"), "B": list("ababc")}) In [112]: pd.get_dummies(df) Out[112]: A_a B_a B_b B_c 0 1 1 0 0 1 1 0 1 0 2 1 1 0 0 3 1 0 1 0 4 1 0 0 1 In [113]: pd.get_dummies(df, drop_first=True) Out[113]: B_b B_c 0 0 0 1 1 0 2 0 0 3 1 0 4 0 1
By default new columns will have np.uint8 dtype. To choose another dtype, use the dtype argument:
np.uint8
dtype
In [114]: df = pd.DataFrame({"A": list("abc"), "B": [1.1, 2.2, 3.3]}) In [115]: pd.get_dummies(df, dtype=bool).dtypes Out[115]: B float64 A_a bool A_b bool A_c bool dtype: object
To encode 1-d values as an enumerated type use factorize():
factorize()
In [116]: x = pd.Series(["A", "A", np.nan, "B", 3.14, np.inf]) In [117]: x Out[117]: 0 A 1 A 2 NaN 3 B 4 3.14 5 inf dtype: object In [118]: labels, uniques = pd.factorize(x) In [119]: labels Out[119]: array([ 0, 0, -1, 1, 2, 3]) In [120]: uniques Out[120]: Index(['A', 'B', 3.14, inf], dtype='object')
Note that factorize is similar to numpy.unique, but differs in its handling of NaN:
factorize
numpy.unique
The following numpy.unique will fail under Python 3 with a TypeError because of an ordering bug. See also here.
In [1]: x = pd.Series(['A', 'A', np.nan, 'B', 3.14, np.inf]) In [2]: pd.factorize(x, sort=True) Out[2]: (array([ 2, 2, -1, 3, 0, 1]), Index([3.14, inf, 'A', 'B'], dtype='object')) In [3]: np.unique(x, return_inverse=True)[::-1] Out[3]: (array([3, 3, 0, 4, 1, 2]), array([nan, 3.14, inf, 'A', 'B'], dtype=object))
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.
df["cat_col"] = pd.Categorical(df["col"])
df["cat_col"] = df["col"].astype("category")
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 [121]: np.random.seed([3, 1415]) In [122]: n = 20 In [123]: cols = np.array(["key", "row", "item", "col"]) In [124]: df = cols + pd.DataFrame( .....: (np.random.randint(5, size=(n, 4)) // [2, 1, 2, 1]).astype(str) .....: ) .....: In [125]: df.columns = cols In [126]: df = df.join(pd.DataFrame(np.random.rand(n, 2).round(2)).add_prefix("val")) In [127]: df Out[127]: 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]
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:
df
col
row
val0
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.
aggfunc='mean'
In [128]: df.pivot_table(values="val0", index="row", columns="col", aggfunc="mean") Out[128]: 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 [129]: df.pivot_table( .....: values="val0", .....: index="row", .....: columns="col", .....: aggfunc="mean", .....: fill_value=0, .....: ) .....: Out[129]: 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.
sum
In [130]: df.pivot_table( .....: values="val0", .....: index="row", .....: columns="col", .....: aggfunc="sum", .....: fill_value=0, .....: ) .....: Out[130]: 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.
size
In [131]: df.pivot_table(index="row", columns="col", fill_value=0, aggfunc="size") Out[131]: 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
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.
mean
In [132]: df.pivot_table( .....: values="val0", .....: index="row", .....: columns="col", .....: aggfunc=["mean", "sum"], .....: ) .....: Out[132]: 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 [133]: df.pivot_table( .....: values=["val0", "val1"], .....: index="row", .....: columns="col", .....: aggfunc=["mean"], .....: ) .....: Out[133]: 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 [134]: df.pivot_table( .....: values=["val0"], .....: index="row", .....: columns=["item", "col"], .....: aggfunc=["mean"], .....: ) .....: Out[134]: 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
New in version 0.25.0.
Sometimes the values in a column are list-like.
In [135]: keys = ["panda1", "panda2", "panda3"] In [136]: values = [["eats", "shoots"], ["shoots", "leaves"], ["eats", "leaves"]] In [137]: df = pd.DataFrame({"keys": keys, "values": values}) In [138]: df Out[138]: 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:
explode()
In [139]: df["values"].explode() Out[139]: 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 [140]: df.explode("values") Out[140]: 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.
Series.explode()
np.nan
In [141]: s = pd.Series([[1, 2, 3], "foo", [], ["a", "b"]]) In [142]: s Out[142]: 0 [1, 2, 3] 1 foo 2 [] 3 [a, b] dtype: object In [143]: s.explode() Out[143]: 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 [144]: df = pd.DataFrame([{"var1": "a,b,c", "var2": 1}, {"var1": "d,e,f", "var2": 2}]) In [145]: df Out[145]: 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 [146]: df.assign(var1=df.var1.str.split(",")).explode("var1") Out[146]: var1 var2 0 a 1 0 b 1 0 c 1 1 d 2 1 e 2 1 f 2