Remove ends of string entries in pandas DataFrame column

I think you can use str.replace with regex .txt$' ( $ – matches the end of the string):

import pandas as pd

df = pd.DataFrame({'A': {0: 2, 1: 1}, 
                   'C': {0: 5, 1: 1}, 
                   'B': {0: 4, 1: 2}, 
                   'filename': {0: "txt.txt", 1: "x.txt"}}, 
                columns=['filename','A','B', 'C'])

print df
  filename  A  B  C
0  txt.txt  2  4  5
1    x.txt  1  2  1

df['filename'] = df['filename'].str.replace(r'.txt$', '')
print df
  filename  A  B  C
0      txt  2  4  5
1        x  1  2  1

df['filename'] = df['filename'].map(lambda x: str(x)[:-4])
print df
  filename  A  B  C
0      txt  2  4  5
1        x  1  2  1

df['filename'] = df['filename'].str[:-4]
print df
  filename  A  B  C
0      txt  2  4  5
1        x  1  2  1

EDIT:

rstrip can remove more characters, if the end of strings contains some characters of striped string (in this case ., t, x):

Example:

print df
  filename  A  B  C
0  txt.txt  2  4  5
1    x.txt  1  2  1

df['filename'] = df['filename'].str.rstrip('.txt')

print df
  filename  A  B  C
0           2  4  5
1           1  2  1