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In this section, we will discuss missing (also referred to as NA) values in pandas.
Note
The choice of using NaN internally to denote missing data was largely
for simplicity and performance reasons. It differs from the MaskedArray
approach of, for example, :mod:`scikits.timeseries`. We are hopeful that
NumPy will soon be able to provide a native NA type solution (similar to R)
performant enough to be used in pandas.
See the :ref:`cookbook<cookbook.missing_data>` for some advanced strategies.
As data comes in many shapes and forms, pandas aims to be flexible with regard
to handling missing data. While NaN is the default missing value marker for
reasons of computational speed and convenience, we need to be able to easily
detect this value with data of different types: floating point, integer,
boolean, and general object. In many cases, however, the Python None will
arise and we wish to also consider that "missing" or "not available" or "NA".
Note
If you want to consider inf and -inf to be "NA" in computations,
you can set pandas.options.mode.use_inf_as_na = True.
.. ipython:: python
df = pd.DataFrame(np.random.randn(5, 3), index=['a', 'c', 'e', 'f', 'h'],
columns=['one', 'two', 'three'])
df['four'] = 'bar'
df['five'] = df['one'] > 0
df
df2 = df.reindex(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'])
df2
To make detecting missing values easier (and across different array dtypes), pandas provides the :func:`isna` and :func:`notna` functions, which are also methods on Series and DataFrame objects:
.. ipython:: python df2['one'] pd.isna(df2['one']) df2['four'].notna() df2.isna()
Warning
One has to be mindful that in Python (and NumPy), the nan's don't compare equal, but None's do.
Note that pandas/NumPy uses the fact that np.nan != np.nan, and treats None like np.nan.
.. ipython:: python None == None np.nan == np.nan
So as compared to above, a scalar equality comparison versus a None/np.nan doesn't provide useful information.
.. ipython:: python df2['one'] == np.nan
Because NaN is a float, a column of integers with even one missing values
is cast to floating-point dtype (see :ref:`gotchas.intna` for more). Pandas
provides a nullable integer array, which can be used by explicitly requesting
the dtype:
.. ipython:: python pd.Series([1, 2, np.nan, 4], dtype=pd.Int64Dtype())
Alternatively, the string alias dtype='Int64' (note the capital "I") can be
used.
See :ref:`integer_na` for more.
For datetime64[ns] types, NaT represents missing values. This is a pseudo-native
sentinel value that can be represented by NumPy in a singular dtype (datetime64[ns]).
pandas objects provide compatibility between NaT and NaN.
.. ipython:: python
df2 = df.copy()
df2['timestamp'] = pd.Timestamp('20120101')
df2
df2.loc[['a', 'c', 'h'], ['one', 'timestamp']] = np.nan
df2
df2.dtypes.value_counts()
You can insert missing values by simply assigning to containers. The actual missing value used will be chosen based on the dtype.
For example, numeric containers will always use NaN regardless of
the missing value type chosen:
.. ipython:: python s = pd.Series([1, 2, 3]) s.loc[0] = None s
Likewise, datetime containers will always use NaT.
For object containers, pandas will use the value given:
.. ipython:: python s = pd.Series(["a", "b", "c"]) s.loc[0] = None s.loc[1] = np.nan s
Missing values propagate naturally through arithmetic operations between pandas objects.
.. ipython:: python :suppress: df = df2.loc[:, ['one', 'two', 'three']] a = df2.loc[df2.index[:5], ['one', 'two']].fillna(method='pad') b = df2.loc[df2.index[:5], ['one', 'two', 'three']]
.. ipython:: python a b a + b
The descriptive statistics and computational methods discussed in the :ref:`data structure overview <basics.stats>` (and listed :ref:`here <api.series.stats>` and :ref:`here <api.dataframe.stats>`) are all written to account for missing data. For example:
- When summing data, NA (missing) values will be treated as zero.
- If the data are all NA, the result will be 0.
- Cumulative methods like :meth:`~DataFrame.cumsum` and :meth:`~DataFrame.cumprod` ignore NA values by default, but preserve them in the resulting arrays. To override this behaviour and include NA values, use
skipna=False.
.. ipython:: python df df['one'].sum() df.mean(1) df.cumsum() df.cumsum(skipna=False)
Warning
This behavior is now standard as of v0.22.0 and is consistent with the default in numpy; previously sum/prod of all-NA or empty Series/DataFrames would return NaN.
See :ref:`v0.22.0 whatsnew <whatsnew_0220>` for more.
The sum of an empty or all-NA Series or column of a DataFrame is 0.
.. ipython:: python pd.Series([np.nan]).sum() pd.Series([]).sum()
The product of an empty or all-NA Series or column of a DataFrame is 1.
.. ipython:: python pd.Series([np.nan]).prod() pd.Series([]).prod()
NA groups in GroupBy are automatically excluded. This behavior is consistent with R, for example:
.. ipython:: python
df
df.groupby('one').mean()
See the groupby section :ref:`here <groupby.missing>` for more information.
pandas objects are equipped with various data manipulation methods for dealing with missing data.
:meth:`~DataFrame.fillna` can "fill in" NA values with non-NA data in a couple of ways, which we illustrate:
Replace NA with a scalar value
.. ipython:: python
df2
df2.fillna(0)
df2['one'].fillna('missing')
Fill gaps forward or backward
Using the same filling arguments as :ref:`reindexing <basics.reindexing>`, we can propagate non-NA values forward or backward:
.. ipython:: python df df.fillna(method='pad')
Limit the amount of filling
If we only want consecutive gaps filled up to a certain number of data points, we can use the limit keyword:
.. ipython:: python :suppress: df.iloc[2:4, :] = np.nan
.. ipython:: python df df.fillna(method='pad', limit=1)
To remind you, these are the available filling methods:
| Method | Action |
|---|---|
| pad / ffill | Fill values forward |
| bfill / backfill | Fill values backward |
With time series data, using pad/ffill is extremely common so that the "last known value" is available at every time point.
:meth:`~DataFrame.ffill` is equivalent to fillna(method='ffill')
and :meth:`~DataFrame.bfill` is equivalent to fillna(method='bfill')
You can also fillna using a dict or Series that is alignable. The labels of the dict or index of the Series must match the columns of the frame you wish to fill. The use case of this is to fill a DataFrame with the mean of that column.
.. ipython:: python
dff = pd.DataFrame(np.random.randn(10, 3), columns=list('ABC'))
dff.iloc[3:5, 0] = np.nan
dff.iloc[4:6, 1] = np.nan
dff.iloc[5:8, 2] = np.nan
dff
dff.fillna(dff.mean())
dff.fillna(dff.mean()['B':'C'])
Same result as above, but is aligning the 'fill' value which is a Series in this case.
.. ipython:: python
dff.where(pd.notna(dff), dff.mean(), axis='columns')
You may wish to simply exclude labels from a data set which refer to missing data. To do this, use :meth:`~DataFrame.dropna`:
.. ipython:: python :suppress: df['two'] = df['two'].fillna(0) df['three'] = df['three'].fillna(0)
.. ipython:: python df df.dropna(axis=0) df.dropna(axis=1) df['one'].dropna()
An equivalent :meth:`~Series.dropna` is available for Series. DataFrame.dropna has considerably more options than Series.dropna, which can be examined :ref:`in the API <api.dataframe.missing>`.
.. versionadded:: 0.23.0 The ``limit_area`` keyword argument was added.
Both Series and DataFrame objects have :meth:`~DataFrame.interpolate` that, by default, performs linear interpolation at missing data points.
.. ipython:: python
:suppress:
np.random.seed(123456)
idx = pd.date_range('1/1/2000', periods=100, freq='BM')
ts = pd.Series(np.random.randn(100), index=idx)
ts[1:5] = np.nan
ts[20:30] = np.nan
ts[60:80] = np.nan
ts = ts.cumsum()
.. ipython:: python ts ts.count() @savefig series_before_interpolate.png ts.plot()
.. ipython:: python ts.interpolate() ts.interpolate().count() @savefig series_interpolate.png ts.interpolate().plot()
Index aware interpolation is available via the method keyword:
.. ipython:: python :suppress: ts2 = ts[[0, 1, 30, 60, 99]]
.. ipython:: python ts2 ts2.interpolate() ts2.interpolate(method='time')
For a floating-point index, use method='values':
.. ipython:: python :suppress: idx = [0., 1., 10.] ser = pd.Series([0., np.nan, 10.], idx)
.. ipython:: python ser ser.interpolate() ser.interpolate(method='values')
You can also interpolate with a DataFrame:
.. ipython:: python
df = pd.DataFrame({'A': [1, 2.1, np.nan, 4.7, 5.6, 6.8],
'B': [.25, np.nan, np.nan, 4, 12.2, 14.4]})
df
df.interpolate()
The method argument gives access to fancier interpolation methods.
If you have scipy installed, you can pass the name of a 1-d interpolation routine to method.
You'll want to consult the full scipy interpolation documentation and reference guide for details.
The appropriate interpolation method will depend on the type of data you are working with.
- If you are dealing with a time series that is growing at an increasing rate,
method='quadratic'may be appropriate. - If you have values approximating a cumulative distribution function,
then
method='pchip'should work well. - To fill missing values with goal of smooth plotting, consider
method='akima'.
Warning
These methods require scipy.
.. ipython:: python df.interpolate(method='barycentric') df.interpolate(method='pchip') df.interpolate(method='akima')
When interpolating via a polynomial or spline approximation, you must also specify the degree or order of the approximation:
.. ipython:: python df.interpolate(method='spline', order=2) df.interpolate(method='polynomial', order=2)
Compare several methods:
.. ipython:: python
np.random.seed(2)
ser = pd.Series(np.arange(1, 10.1, .25) ** 2 + np.random.randn(37))
missing = np.array([4, 13, 14, 15, 16, 17, 18, 20, 29])
ser[missing] = np.nan
methods = ['linear', 'quadratic', 'cubic']
df = pd.DataFrame({m: ser.interpolate(method=m) for m in methods})
@savefig compare_interpolations.png
df.plot()
Another use case is interpolation at new values.
Suppose you have 100 observations from some distribution. And let's suppose
that you're particularly interested in what's happening around the middle.
You can mix pandas' reindex and interpolate methods to interpolate
at the new values.
.. ipython:: python ser = pd.Series(np.sort(np.random.uniform(size=100))) # interpolate at new_index new_index = ser.index | pd.Index([49.25, 49.5, 49.75, 50.25, 50.5, 50.75]) interp_s = ser.reindex(new_index).interpolate(method='pchip') interp_s[49:51]
Like other pandas fill methods, :meth:`~DataFrame.interpolate` accepts a limit keyword
argument. Use this argument to limit the number of consecutive NaN values
filled since the last valid observation:
.. ipython:: python
ser = pd.Series([np.nan, np.nan, 5, np.nan, np.nan,
np.nan, 13, np.nan, np.nan])
ser
# fill all consecutive values in a forward direction
ser.interpolate()
# fill one consecutive value in a forward direction
ser.interpolate(limit=1)
By default, NaN values are filled in a forward direction. Use
limit_direction parameter to fill backward or from both directions.
.. ipython:: python # fill one consecutive value backwards ser.interpolate(limit=1, limit_direction='backward') # fill one consecutive value in both directions ser.interpolate(limit=1, limit_direction='both') # fill all consecutive values in both directions ser.interpolate(limit_direction='both')
By default, NaN values are filled whether they are inside (surrounded by)
existing valid values, or outside existing valid values. Introduced in v0.23
the limit_area parameter restricts filling to either inside or outside values.
.. ipython:: python # fill one consecutive inside value in both directions ser.interpolate(limit_direction='both', limit_area='inside', limit=1) # fill all consecutive outside values backward ser.interpolate(limit_direction='backward', limit_area='outside') # fill all consecutive outside values in both directions ser.interpolate(limit_direction='both', limit_area='outside')
Often times we want to replace arbitrary values with other values.
:meth:`~Series.replace` in Series and :meth:`~DataFrame.replace` in DataFrame provides an efficient yet flexible way to perform such replacements.
For a Series, you can replace a single value or a list of values by another value:
.. ipython:: python ser = pd.Series([0., 1., 2., 3., 4.]) ser.replace(0, 5)
You can replace a list of values by a list of other values:
.. ipython:: python ser.replace([0, 1, 2, 3, 4], [4, 3, 2, 1, 0])
You can also specify a mapping dict:
.. ipython:: python
ser.replace({0: 10, 1: 100})
For a DataFrame, you can specify individual values by column:
.. ipython:: python
df = pd.DataFrame({'a': [0, 1, 2, 3, 4], 'b': [5, 6, 7, 8, 9]})
df.replace({'a': 0, 'b': 5}, 100)
Instead of replacing with specified values, you can treat all given values as missing and interpolate over them:
.. ipython:: python ser.replace([1, 2, 3], method='pad')
Note
Python strings prefixed with the r character such as r'hello world'
are so-called "raw" strings. They have different semantics regarding
backslashes than strings without this prefix. Backslashes in raw strings
will be interpreted as an escaped backslash, e.g., r'\' == '\\'. You
should read about them
if this is unclear.
Replace the '.' with NaN (str -> str):
.. ipython:: python
d = {'a': list(range(4)), 'b': list('ab..'), 'c': ['a', 'b', np.nan, 'd']}
df = pd.DataFrame(d)
df.replace('.', np.nan)
Now do it with a regular expression that removes surrounding whitespace (regex -> regex):
.. ipython:: python df.replace(r'\s*\.\s*', np.nan, regex=True)
Replace a few different values (list -> list):
.. ipython:: python df.replace(['a', '.'], ['b', np.nan])
list of regex -> list of regex:
.. ipython:: python df.replace([r'\.', r'(a)'], ['dot', r'\1stuff'], regex=True)
Only search in column 'b' (dict -> dict):
.. ipython:: python
df.replace({'b': '.'}, {'b': np.nan})
Same as the previous example, but use a regular expression for searching instead (dict of regex -> dict):
.. ipython:: python
df.replace({'b': r'\s*\.\s*'}, {'b': np.nan}, regex=True)
You can pass nested dictionaries of regular expressions that use regex=True:
.. ipython:: python
df.replace({'b': {'b': r''}}, regex=True)
Alternatively, you can pass the nested dictionary like so:
.. ipython:: python
df.replace(regex={'b': {r'\s*\.\s*': np.nan}})
You can also use the group of a regular expression match when replacing (dict of regex -> dict of regex), this works for lists as well.
.. ipython:: python
df.replace({'b': r'\s*(\.)\s*'}, {'b': r'\1ty'}, regex=True)
You can pass a list of regular expressions, of which those that match will be replaced with a scalar (list of regex -> regex).
.. ipython:: python df.replace([r'\s*\.\s*', r'a|b'], np.nan, regex=True)
All of the regular expression examples can also be passed with the
to_replace argument as the regex argument. In this case the value
argument must be passed explicitly by name or regex must be a nested
dictionary. The previous example, in this case, would then be:
.. ipython:: python df.replace(regex=[r'\s*\.\s*', r'a|b'], value=np.nan)
This can be convenient if you do not want to pass regex=True every time you
want to use a regular expression.
Note
Anywhere in the above replace examples that you see a regular expression
a compiled regular expression is valid as well.
:meth:`~DataFrame.replace` is similar to :meth:`~DataFrame.fillna`.
.. ipython:: python df = pd.DataFrame(np.random.randn(10, 2)) df[np.random.rand(df.shape[0]) > 0.5] = 1.5 df.replace(1.5, np.nan)
Replacing more than one value is possible by passing a list.
.. ipython:: python df00 = df.iloc[0, 0] df.replace([1.5, df00], [np.nan, 'a']) df[1].dtype
You can also operate on the DataFrame in place:
.. ipython:: python df.replace(1.5, np.nan, inplace=True)
Warning
When replacing multiple bool or datetime64 objects, the first
argument to replace (to_replace) must match the type of the value
being replaced. For example,
>>> s = pd.Series([True, False, True])
>>> s.replace({'a string': 'new value', True: False}) # raises
TypeError: Cannot compare types 'ndarray(dtype=bool)' and 'str'will raise a TypeError because one of the dict keys is not of the
correct type for replacement.
However, when replacing a single object such as,
.. ipython:: python
s = pd.Series([True, False, True])
s.replace('a string', 'another string')
the original NDFrame object will be returned untouched. We're working on
unifying this API, but for backwards compatibility reasons we cannot break
the latter behavior. See :issue:`6354` for more details.
While pandas supports storing arrays of integer and boolean type, these types are not capable of storing missing data. Until we can switch to using a native NA type in NumPy, we've established some "casting rules". When a reindexing operation introduces missing data, the Series will be cast according to the rules introduced in the table below.
| data type | Cast to |
|---|---|
| integer | float |
| boolean | object |
| float | no cast |
| object | no cast |
For example:
.. ipython:: python s = pd.Series(np.random.randn(5), index=[0, 2, 4, 6, 7]) s > 0 (s > 0).dtype crit = (s > 0).reindex(list(range(8))) crit crit.dtype
Ordinarily NumPy will complain if you try to use an object array (even if it contains boolean values) instead of a boolean array to get or set values from an ndarray (e.g. selecting values based on some criteria). If a boolean vector contains NAs, an exception will be generated:
.. ipython:: python :okexcept: reindexed = s.reindex(list(range(8))).fillna(0) reindexed[crit]
However, these can be filled in using :meth:`~DataFrame.fillna` and it will work fine:
.. ipython:: python reindexed[crit.fillna(False)] reindexed[crit.fillna(True)]
Pandas provides a nullable integer dtype, but you must explicitly request it
when creating the series or column. Notice that we use a capital "I" in
the dtype="Int64".
.. ipython:: python s = pd.Series([0, 1, np.nan, 3, 4], dtype="Int64") s
See :ref:`integer_na` for more.