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’s memory usage will be displayed when invoking the df.info() method.
DataFrame
info()
display.memory_usage
df.info()
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: 289.1+ 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.
+
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.
memory_usage='deep'
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: 425.6 KB
By default the display option is set to True but can be explicitly overridden by passing the memory_usage argument when invoking df.info().
True
memory_usage
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:
memory_usage()
Series
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 10920 dtype: int64 # total memory usage of dataframe In [9]: df.memory_usage().sum() Out[9]: 296048
By default the memory usage of the DataFrame’s index is shown in the returned Series, the memory usage of the index can be suppressed by passing the index=False argument:
index=False
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 10920 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.
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:
bool
if
and
or
not
>>> 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:
False
ValueError
>>> if pd.Series([False, True, False]): ... print("I was true") Traceback ... ValueError: The truth value of an array is ambiguous. Use a.empty, 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:
any()
all()
empty()
None
>>> 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:
>>> if pd.Series([False, True, False]).any(): ... print("I am any") I am any
To evaluate single-element pandas objects in a boolean context, use the method bool():
bool()
In [11]: pd.Series([True]).bool() Out[11]: True In [12]: pd.Series([False]).bool() Out[12]: False In [13]: pd.DataFrame([[True]]).bool() Out[13]: True In [14]: pd.DataFrame([[False]]).bool() Out[14]: False
Bitwise boolean operators like == and != return a boolean Series, which is almost always what you want anyways.
==
!=
>>> s = pd.Series(range(5)) >>> s == 4 0 False 1 False 2 False 3 False 4 True dtype: bool
See boolean comparisons for more examples.
in
Using the Python in operator on a Series tests for membership in the index, not membership among the values.
In [15]: s = pd.Series(range(5), index=list('abcde')) In [16]: 2 in s Out[16]: False In [17]: 'b' in s Out[17]: 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():
isin()
In [18]: s.isin([2]) Out[18]: a False b False c True d False e False dtype: bool In [19]: s.isin([2]).any() Out[19]: True
For DataFrames, likewise, in applies to the column axis, testing for membership in the list of column names.
DataFrames
NaN
NA
For lack of NA (missing) support from the ground up in NumPy and Python in general, we were given the difficult choice between either:
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 NA across the dtypes.
For many reasons we chose the latter. After years of production use it has proven, at least in my opinion, to be the best decision given the state of affairs in NumPy and Python in general. The special value NaN (Not-A-Number) is used everywhere as the NA value, and there are API functions isna and notna which can be used across the dtypes to detect NA values.
isna
notna
However, it comes with it a couple of trade-offs which I most certainly have not ignored.
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 [20]: s = pd.Series([1, 2, 3, 4, 5], index=list('abcde')) In [21]: s Out[21]: a 1 b 2 c 3 d 4 e 5 dtype: int64 In [22]: s.dtype Out[22]: dtype('int64') In [23]: s2 = s.reindex(['a', 'b', 'c', 'f', 'u']) In [24]: s2 Out[24]: a 1.0 b 2.0 c 3.0 f NaN u NaN dtype: float64 In [25]: s2.dtype Out[25]: 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
Int8Dtype
Int16Dtype
Int32Dtype
Int64Dtype
In [26]: s_int = pd.Series([1, 2, 3, 4, 5], index=list('abcde'), ....: dtype=pd.Int64Dtype()) ....: In [27]: s_int Out[27]: a 1 b 2 c 3 d 4 e 5 dtype: Int64 In [28]: s_int.dtype Out[28]: Int64Dtype() In [29]: s2_int = s_int.reindex(['a', 'b', 'c', 'f', 'u']) In [30]: s2_int Out[30]: a 1 b 2 c 3 f <NA> u <NA> dtype: Int64 In [31]: s2_int.dtype Out[31]: Int64Dtype()
See Nullable integer data type for more.
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:
reindex()
Typeclass
Promotion dtype for storing NAs
floating
no change
object
integer
cast to float64
float64
boolean
cast to object
While this may seem like a heavy trade-off, I have found very few cases where this is an issue in practice i.e. storing values greater than 2**53. Some explanation for the motivation is in the next section.
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:
Dtypes
numpy.floating
float16, float32, float64, float128
numpy.integer
int8, int16, int32, int64
numpy.unsignedinteger
uint8, uint16, uint32, uint64
numpy.object_
object_
numpy.bool_
bool_
numpy.character
string_, unicode_
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.
numeric
character
An alternate approach is that of using masked arrays. A masked array is an array of data with an associated boolean mask denoting whether each value should be considered NA or not. I am personally not in love with this approach as I feel that overall it places a fairly heavy burden on the user and the library implementer. Additionally, it exacts a fairly high performance cost when working with numerical data compared with the simple approach of using NaN. Thus, I have chosen the Pythonic “practicality beats purity” approach and traded integer NA capability for a much simpler approach of using a special value in float and object arrays to denote NA, and promoting integer arrays to floating when NAs must be introduced.
For Series and DataFrame objects, var() normalizes by N-1 to produce unbiased estimates of the sample variance, while NumPy’s var normalizes by N, which measures the variance of the sample. Note that cov() normalizes by N-1 in both pandas and NumPy.
var()
N-1
var
cov()
As of pandas 0.11, 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.
copy()
See this link for more information.
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 [32]: x = np.array(list(range(10)), '>i4') # big endian In [33]: newx = x.byteswap().newbyteorder() # force native byteorder In [34]: s = pd.Series(newx)
See the NumPy documentation on byte order for more details.