Working with missing data¶
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, 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 cookbook for some advanced strategies.
Missing data basics¶
When / why does data become missing?¶
Some might quibble over our usage of missing. By “missing” we simply mean NA (“not available”) or “not present for whatever reason”. Many data sets simply arrive with missing data, either because it exists and was not collected or it never existed. For example, in a collection of financial time series, some of the time series might start on different dates. Thus, values prior to the start date would generally be marked as missing.
In pandas, one of the most common ways that missing data is introduced into a data set is by reindexing. For example:
In [1]: df = pd.DataFrame(np.random.randn(5, 3), index=['a', 'c', 'e', 'f', 'h'],
...: columns=['one', 'two', 'three'])
...:
In [2]: df['four'] = 'bar'
In [3]: df['five'] = df['one'] > 0
In [4]: df
Out[4]:
one two three four five
a -0.166778 0.501113 -0.355322 bar False
c -0.337890 0.580967 0.983801 bar False
e 0.057802 0.761948 -0.712964 bar True
f -0.443160 -0.974602 1.047704 bar False
h -0.717852 -1.053898 -0.019369 bar False
In [5]: df2 = df.reindex(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'])
In [6]: df2
Out[6]:
one two three four five
a -0.166778 0.501113 -0.355322 bar False
b NaN NaN NaN NaN NaN
c -0.337890 0.580967 0.983801 bar False
d NaN NaN NaN NaN NaN
e 0.057802 0.761948 -0.712964 bar True
f -0.443160 -0.974602 1.047704 bar False
g NaN NaN NaN NaN NaN
h -0.717852 -1.053898 -0.019369 bar False
Values considered “missing”¶
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
.
To make detecting missing values easier (and across different array dtypes),
pandas provides the isna()
and
notna()
functions, which are also methods on
Series and DataFrame objects:
In [7]: df2['one']
Out[7]:
a -0.166778
b NaN
c -0.337890
d NaN
e 0.057802
f -0.443160
g NaN
h -0.717852
Name: one, dtype: float64
In [8]: pd.isna(df2['one'])