python - How to treat NULL as a normal string with pandas?

Question

I have a csv-file with a column with strings and I want to read it with pandas. In this file the string null occurs as an actual value and should not be regarded as a missing value.

Example:

import pandas as pd
from io import StringIO

data = u'strings,numbers
foo,1
bar,2
null,3'
print(pd.read_csv(StringIO(data)))

This gives the following output:

  strings  numbers
0     foo        1
1     bar        2
2     NaN        3

What can I do to get the value null as it is (and not as NaN) into the DataFrame? The file can be assumed to not contain any actually missing values.

Answer 1

You can specify a converters argument for the string column.

pd.read_csv(StringIO(data), converters={'strings' : str})

  strings  numbers
0     foo        1
1     bar        2
2    null        3

This will by-pass pandas' automatic parsing.

---

Another option is setting na_filter=False:

pd.read_csv(StringIO(data), na_filter=False)

  strings  numbers
0     foo        1
1     bar        2
2    null        3

This works for the entire DataFrame, so use with caution. I recommend first option if you want to surgically apply this to select columns instead.