How to replace values with None in Pandas data frame in Python

Question

Is there any method to replace values with None in Pandas in Python?

You can use df.replace('pre', 'post') and can replace a value with another, but this can't be done if you want to replace with None value, which if you try, you get a strange result.

So here's an example:

df = DataFrame(['-',3,2,5,1,-5,-1,'-',9])
df.replace('-', 0)

which returns a successful result.

But,

df.replace('-', None)

which returns a following result:

0
0   - // this isn't replaced
1   3
2   2
3   5
4   1
5  -5
6  -1
7  -1 // this is changed to `-1`...
8   9

Why does such a strange result be returned?

Since I want to pour this data frame into MySQL database, I can't put NaN values into any element in my data frame and instead want to put None. Surely, you can first change '-' to NaN and then convert NaN to None, but I want to know why the dataframe acts in such a terrible way.

Answer 1

Actually in later versions of pandas this will give a TypeError:

df.replace('-', None)
TypeError: If "to_replace" and "value" are both None then regex must be a mapping

You can do it by passing either a list or a dictionary:

In [11]: df.replace('-', df.replace(['-'], [None]) # or .replace('-', {0: None})
Out[11]:
      0
0  None
1     3
2     2
3     5
4     1
5    -5
6    -1
7  None
8     9

But I recommend using NaNs rather than None:

In [12]: df.replace('-', np.nan)
Out[12]:
     0
0  NaN
1    3
2    2
3    5
4    1
5   -5
6   -1
7  NaN
8    9