为什么测试`NaN == NaN`不能从pandas dataFrame中删除？

Question

Please explain how NaN's are treated in pandas because the following logic seems "broken" to me, I tried various ways (shown below) to drop the empty values.

My dataframe, which I load from a CSV file using read.csv , has a column comments , which is empty most of the time.

The column marked_results.comments looks like this; all the rest of the column is NaN, so pandas loads empty entries as NaNs, so far so good:

0       VP
1       VP
2       VP
3     TEST
4      NaN
5      NaN
....

Now I try to drop those entries, only this works:

• marked_results.comments.isnull()

All these don't work:

• marked_results.comments.dropna() only gives the same column, nothing gets dropped, confusing.
• marked_results.comments == NaN only gives a series of all False s. Nothing was NaNs... confusing.
• likewise marked_results.comments == nan

---

I also tried:

comments_values = marked_results.comments.unique()

array(['VP', 'TEST', nan], dtype=object)

# Ah, gotya! so now ive tried:
marked_results.comments == comments_values[2]
# but still all the results are Falses!!!

Answer 1

You should use isnull and notnull to test for NaN (these are more robust using pandas dtypes than numpy), see "values considered missing" in the docs .

Using the Series method dropna on a column won't affect the original dataframe, but do what you want:

In [11]: df
Out[11]:
  comments
0       VP
1       VP
2       VP
3     TEST
4      NaN
5      NaN

In [12]: df.comments.dropna()
Out[12]:
0      VP
1      VP
2      VP
3    TEST
Name: comments, dtype: object

The dropna DataFrame method has a subset argument (to drop rows which have NaNs in specific columns):

In [13]: df.dropna(subset=['comments'])
Out[13]:
  comments
0       VP
1       VP
2       VP
3     TEST

In [14]: df = df.dropna(subset=['comments'])

Answer 2

You need to test NaN with math.isnan() function (Or numpy.isnan ). NaNs cannot be checked with the equality operator.

>>> a = float('NaN')
>>> a
nan
>>> a == 'NaN'
False
>>> isnan(a)
True
>>> a == float('NaN')
False

Help Function ->

isnan(...)
    isnan(x) -> bool

    Check if float x is not a number (NaN).