Frequently Asked Questions (FAQ)#
DataFrame memory usage#
The memory usage of a DataFrame (including the index) is shown when calling
the info(). A configuration option, display.memory_usage
(see the list of options), specifies if the
DataFrame memory usage will be displayed when invoking the info()
method.
For example, the memory usage of the DataFrame below is shown
when calling info():
In [1]: dtypes = [
...: "int64",
...: "float64",
...: "datetime64[ns]",
...: "timedelta64[ns]",
...: "complex128",
...: "object",
...: "bool",
...: ]
...:
In [2]: n = 5000
In [3]: data = {t: np.random.randint(100, size=n).astype(t) for t in dtypes}
In [4]: df = pd.DataFrame(data)
In [5]: df["categorical"] = df["object"].astype("category")
In [6]: df.info()
<class 'pandas.core.frame.DataFrame'>
RangeIndex: 5000 entries, 0 to 4999
Data columns (total 8 columns):
# Column Non-Null Count Dtype
--- ------ -------------- -----
0 int64 5000 non-null int64
1 float64 5000 non-null float64
2 datetime64[ns] 5000 non-null datetime64[ns]
3 timedelta64[ns] 5000 non-null timedelta64[ns]
4 complex128 5000 non-null complex128
5 object 5000 non-null object
6 bool 5000 non-null bool
7 categorical 5000 non-null category
dtypes: bool(1), category(1), complex128(1), datetime64[ns](1), float64(1), int64(1), object(1), timedelta64[ns](1)
memory usage: 288.2+ KB
The + symbol indicates that the true memory usage could be higher, because
pandas does not count the memory used by values in columns with
dtype=object.
Passing memory_usage='deep' will enable a more accurate memory usage report,
accounting for the full usage of the contained objects. This is optional
as it can be expensive to do this deeper introspection.
In [7]: df.info(memory_usage="deep")
<class 'pandas.core.frame.DataFrame'>
RangeIndex: 5000 entries, 0 to 4999
Data columns (total 8 columns):
# Column Non-Null Count Dtype
--- ------ -------------- -----
0 int64 5000 non-null int64
1 float64 5000 non-null float64
2 datetime64[ns] 5000 non-null datetime64[ns]
3 timedelta64[ns] 5000 non-null timedelta64[ns]
4 complex128 5000 non-null complex128
5 object 5000 non-null object
6 bool 5000 non-null bool
7 categorical 5000 non-null category
dtypes: bool(1), category(1), complex128(1), datetime64[ns](1), float64(1), int64(1), object(1), timedelta64[ns](1)
memory usage: 424.7 KB
By default the display option is set to True but can be explicitly
overridden by passing the memory_usage argument when invoking info().
The memory usage of each column can be found by calling the
memory_usage() method. This returns a Series with an index
represented by column names and memory usage of each column shown in bytes. For
the DataFrame above, the memory usage of each column and the total memory
usage can be found with the memory_usage() method:
In [8]: df.memory_usage()
Out[8]:
Index 128
int64 40000
float64 40000
datetime64[ns] 40000
timedelta64[ns] 40000
complex128 80000
object 40000
bool 5000
categorical 9968
dtype: int64
# total memory usage of dataframe
In [9]: df.memory_usage().sum()
Out[9]: 295096
By default the memory usage of the DataFrame index is shown in the
returned Series, the memory usage of the index can be suppressed by passing
the index=False argument:
In [10]: df.memory_usage(index=False)
Out[10]:
int64 40000
float64 40000
datetime64[ns] 40000
timedelta64[ns] 40000
complex128 80000
object 40000
bool 5000
categorical 9968
dtype: int64
The memory usage displayed by the info() method utilizes the
memory_usage() method to determine the memory usage of a
DataFrame while also formatting the output in human-readable units (base-2
representation; i.e. 1KB = 1024 bytes).
See also Categorical Memory Usage.
Using if/truth statements with pandas#
pandas follows the NumPy convention of raising an error when you try to convert
something to a bool. This happens in an if-statement or when using the
boolean operations: and, or, and not. It is not clear what the result
of the following code should be:
>>> if pd.Series([False, True, False]):
... pass
Should it be True because it’s not zero-length, or False because there
are False values? It is unclear, so instead, pandas raises a ValueError:
In [11]: if pd.Series([False, True, False]):
....: print("I was true")
....:
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-11-5c782b38cd2f> in ?()
----> 1 if pd.Series([False, True, False]):
2 print("I was true")
~/work/pandas/pandas/pandas/core/generic.py in ?(self)
1517 @final
1518 def __nonzero__(self) -> NoReturn:
-> 1519 raise ValueError(
1520 f"The truth value of a {type(self).__name__} is ambiguous. "
1521 "Use a.empty, a.bool(), a.item(), a.any() or a.all()."
1522 )
ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
You need to explicitly choose what you want to do with the DataFrame, e.g.
use any(), all() or empty().
Alternatively, you might want to compare if the pandas object is None:
In [12]: if pd.Series([False, True, False]) is not None:
....: print("I was not None")
....:
I was not None
Below is how to check if any of the values are True:
In [13]: if pd.Series([False, True, False]).any():
....: print("I am any")
....:
I am any
Bitwise boolean#
Bitwise boolean operators like == and != return a boolean Series
which performs an element-wise comparison when compared to a scalar.
In [14]: s = pd.Series(range(5))
In [15]: s == 4
Out[15]:
0 False
1 False
2 False
3 False
4 True
dtype: bool
See boolean comparisons for more examples.
Using the in operator#
Using the Python in operator on a Series tests for membership in the
index, not membership among the values.
In [16]: s = pd.Series(range(5), index=list("abcde"))
In [17]: 2 in s
Out[17]: False
In [18]: 'b' in s
Out[18]: True
If this behavior is surprising, keep in mind that using in on a Python
dictionary tests keys, not values, and Series are dict-like.
To test for membership in the values, use the method isin():
In [19]: s.isin([2])
Out[19]:
a False
b False
c True
d False
e False
dtype: bool
In [20]: s.isin([2]).any()
Out[20]: True
For DataFrame, likewise, in applies to the column axis,
testing for membership in the list of column names.
Mutating with User Defined Function (UDF) methods#
This section applies to pandas methods that take a UDF. In particular, the methods
DataFrame.apply(), DataFrame.aggregate(), DataFrame.transform(), and
DataFrame.filter().
It is a general rule in programming that one should not mutate a container while it is being iterated over. Mutation will invalidate the iterator, causing unexpected behavior. Consider the example:
In [21]: values = [0, 1, 2, 3, 4, 5]
In [22]: n_removed = 0
In [23]: for k, value in enumerate(values):
....: idx = k - n_removed
....: if value % 2 == 1:
....: del values[idx]
....: n_removed += 1
....: else:
....: values[idx] = value + 1
....:
In [24]: values
Out[24]: [1, 4, 5]
One probably would have expected that the result would be [1, 3, 5].
When using a pandas method that takes a UDF, internally pandas is often
iterating over the
DataFrame or other pandas object. Therefore, if the UDF mutates (changes)
the DataFrame, unexpected behavior can arise.
Here is a similar example with DataFrame.apply():
In [25]: def f(s):
....: s.pop("a")
....: return s
....:
In [26]: df = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
In [27]: df.apply(f, axis="columns")
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
File ~/work/pandas/pandas/pandas/core/indexes/base.py:3790, in Index.get_loc(self, key)
3789 try:
-> 3790 return self._engine.get_loc(casted_key)
3791 except KeyError as err:
File index.pyx:152, in pandas._libs.index.IndexEngine.get_loc()
File index.pyx:181, in pandas._libs.index.IndexEngine.get_loc()
File pandas/_libs/hashtable_class_helper.pxi:7080, in pandas._libs.hashtable.PyObjectHashTable.get_item()
File pandas/_libs/hashtable_class_helper.pxi:7088, in pandas._libs.hashtable.PyObjectHashTable.get_item()
KeyError: 'a'
The above exception was the direct cause of the following exception:
KeyError Traceback (most recent call last)
Cell In[27], line 1
----> 1 df.apply(f, axis="columns")
File ~/work/pandas/pandas/pandas/core/frame.py:10037, in DataFrame.apply(self, func, axis, raw, result_type, args, by_row, **kwargs)
10025 from pandas.core.apply import frame_apply
10027 op = frame_apply(
10028 self,
10029 func=func,
(...)
10035 kwargs=kwargs,
10036 )
> 10037 return op.apply().__finalize__(self, method="apply")
File ~/work/pandas/pandas/pandas/core/apply.py:837, in FrameApply.apply(self)
834 elif self.raw:
835 return self.apply_raw()
--> 837 return self.apply_standard()
File ~/work/pandas/pandas/pandas/core/apply.py:963, in FrameApply.apply_standard(self)
962 def apply_standard(self):
--> 963 results, res_index = self.apply_series_generator()
965 # wrap results
966 return self.wrap_results(results, res_index)
File ~/work/pandas/pandas/pandas/core/apply.py:979, in FrameApply.apply_series_generator(self)
976 with option_context("mode.chained_assignment", None):
977 for i, v in enumerate(series_gen):
978 # ignore SettingWithCopy here in case the user mutates
--> 979 results[i] = self.func(v, *self.args, **self.kwargs)
980 if isinstance(results[i], ABCSeries):
981 # If we have a view on v, we need to make a copy because
982 # series_generator will swap out the underlying data
983 results[i] = results[i].copy(deep=False)
Cell In[25], line 2, in f(s)
1 def f(s):
----> 2 s.pop("a")
3 return s
File ~/work/pandas/pandas/pandas/core/series.py:5179, in Series.pop(self, item)
5154 def pop(self, item: Hashable) -> Any:
5155 """
5156 Return item and drops from series. Raise KeyError if not found.
5157
(...)
5177 dtype: int64
5178 """
-> 5179 return super().pop(item=item)
File ~/work/pandas/pandas/pandas/core/generic.py:901, in NDFrame.pop(self, item)
900 def pop(self, item: Hashable) -> Series | Any:
--> 901 result = self[item]
902 del self[item]
904 return result
File ~/work/pandas/pandas/pandas/core/series.py:1040, in Series.__getitem__(self, key)
1037 return self._values[key]
1039 elif key_is_scalar:
-> 1040 return self._get_value(key)
1042 # Convert generator to list before going through hashable part
1043 # (We will iterate through the generator there to check for slices)
1044 if is_iterator(key):
File ~/work/pandas/pandas/pandas/core/series.py:1156, in Series._get_value(self, label, takeable)
1153 return self._values[label]
1155 # Similar to Index.get_value, but we do not fall back to positional
-> 1156 loc = self.index.get_loc(label)
1158 if is_integer(loc):
1159 return self._values[loc]
File ~/work/pandas/pandas/pandas/core/indexes/base.py:3797, in Index.get_loc(self, key)
3792 if isinstance(casted_key, slice) or (
3793 isinstance(casted_key, abc.Iterable)
3794 and any(isinstance(x, slice) for x in casted_key)
3795 ):
3796 raise InvalidIndexError(key)
-> 3797 raise KeyError(key) from err
3798 except TypeError:
3799 # If we have a listlike key, _check_indexing_error will raise
3800 # InvalidIndexError. Otherwise we fall through and re-raise
3801 # the TypeError.
3802 self._check_indexing_error(key)
KeyError: 'a'
To resolve this issue, one can make a copy so that the mutation does not apply to the container being iterated over.
In [28]: values = [0, 1, 2, 3, 4, 5]
In [29]: n_removed = 0
In [30]: for k, value in enumerate(values.copy()):
....: idx = k - n_removed
....: if value % 2 == 1:
....: del values[idx]
....: n_removed += 1
....: else:
....: values[idx] = value + 1
....:
In [31]: values
Out[31]: [1, 3, 5]
In [32]: def f(s):
....: s = s.copy()
....: s.pop("a")
....: return s
....:
In [33]: df = pd.DataFrame({"a": [1, 2, 3], 'b': [4, 5, 6]})
In [34]: df.apply(f, axis="columns")
Out[34]:
b
0 4
1 5
2 6
Missing value representation for NumPy types#
np.nan as the NA representation for NumPy types#
For lack of NA (missing) support from the ground up in NumPy and Python in
general, NA could have been represented with:
A masked array solution: an array of data and an array of boolean values indicating whether a value is there or is missing.
Using a special sentinel value, bit pattern, or set of sentinel values to denote
NAacross the dtypes.
The special value np.nan (Not-A-Number) was chosen as the NA value for NumPy types, and there are API
functions like DataFrame.isna() and DataFrame.notna() which can be used across the dtypes to
detect NA values. However, this choice has a downside of coercing missing integer data as float types as
shown in Support for integer NA.
NA type promotions for NumPy types#
When introducing NAs into an existing Series or DataFrame via
reindex() or some other means, boolean and integer types will be
promoted to a different dtype in order to store the NAs. The promotions are
summarized in this table:
Typeclass |
Promotion dtype for storing NAs |
|---|---|
|
no change |
|
no change |
|
cast to |
|
cast to |
Support for integer NA#
In the absence of high performance NA support being built into NumPy from
the ground up, the primary casualty is the ability to represent NAs in integer
arrays. For example:
In [35]: s = pd.Series([1, 2, 3, 4, 5], index=list("abcde"))
In [36]: s
Out[36]:
a 1
b 2
c 3
d 4
e 5
dtype: int64
In [37]: s.dtype
Out[37]: dtype('int64')
In [38]: s2 = s.reindex(["a", "b", "c", "f", "u"])
In [39]: s2
Out[39]:
a 1.0
b 2.0
c 3.0
f NaN
u NaN
dtype: float64
In [40]: s2.dtype
Out[40]: dtype('float64')
This trade-off is made largely for memory and performance reasons, and also so
that the resulting Series continues to be “numeric”.
If you need to represent integers with possibly missing values, use one of the nullable-integer extension dtypes provided by pandas or pyarrow
In [41]: s_int = pd.Series([1, 2, 3, 4, 5], index=list("abcde"), dtype=pd.Int64Dtype())
In [42]: s_int
Out[42]:
a 1
b 2
c 3
d 4
e 5
dtype: Int64
In [43]: s_int.dtype
Out[43]: Int64Dtype()
In [44]: s2_int = s_int.reindex(["a", "b", "c", "f", "u"])
In [45]: s2_int
Out[45]:
a 1
b 2
c 3
f <NA>
u <NA>
dtype: Int64
In [46]: s2_int.dtype
Out[46]: Int64Dtype()
In [47]: s_int_pa = pd.Series([1, 2, None], dtype="int64[pyarrow]")
In [48]: s_int_pa
Out[48]:
0 1
1 2
2 <NA>
dtype: int64[pyarrow]
See Nullable integer data type and PyArrow Functionality for more.
Why not make NumPy like R?#
Many people have suggested that NumPy should simply emulate the NA support
present in the more domain-specific statistical programming language R. Part of the reason is the NumPy type hierarchy:
Typeclass |
Dtypes |
|---|---|
|
|
|
|
|
|
|
|
|
|
|
|
The R language, by contrast, only has a handful of built-in data types:
integer, numeric (floating-point), character, and
boolean. NA types are implemented by reserving special bit patterns for
each type to be used as the missing value. While doing this with the full NumPy
type hierarchy would be possible, it would be a more substantial trade-off
(especially for the 8- and 16-bit data types) and implementation undertaking.
However, R NA semantics are now available by using masked NumPy types such as Int64Dtype
or PyArrow types (ArrowDtype).
Differences with NumPy#
For Series and DataFrame objects, var() normalizes by
N-1 to produce unbiased estimates of the population variance, while NumPy’s
numpy.var() normalizes by N, which measures the variance of the sample. Note that
cov() normalizes by N-1 in both pandas and NumPy.
Thread-safety#
pandas is not 100% thread safe. The known issues relate to
the copy() method. If you are doing a lot of copying of
DataFrame objects shared among threads, we recommend holding locks inside
the threads where the data copying occurs.
See this link for more information.
Byte-ordering issues#
Occasionally you may have to deal with data that were created on a machine with a different byte order than the one on which you are running Python. A common symptom of this issue is an error like:
Traceback
...
ValueError: Big-endian buffer not supported on little-endian compiler
To deal
with this issue you should convert the underlying NumPy array to the native
system byte order before passing it to Series or DataFrame
constructors using something similar to the following:
In [49]: x = np.array(list(range(10)), ">i4") # big endian
In [50]: newx = x.byteswap().view(x.dtype.newbyteorder()) # force native byteorder
In [51]: s = pd.Series(newx)
See the NumPy documentation on byte order for more details.