1) To remove white space everywhere:
df.columns = df.columns.str.replace(' ', '')
2) To remove white space at the beginning of string:
df.columns = df.columns.str.lstrip()
3) To remove white space at the end of string:
df.columns = df.columns.str.rstrip()
4) To remove white space at both ends:
df.columns = df.columns.str.strip()
5) To replace white space everywhere
df.columns = df.columns.str.replace(' ', '_')
6) To replace white space at the beginning:
df.columns = df.columns.str.replace('^ +', '_')
7) To replace white space at the end:
df.columns = df.columns.str.replace(' +$', '_')
8) To replace white space at both ends:
df.columns = df.columns.str.replace('^ +| +$', '_')
All above applies to a specific column as well, assume you have a column named col, then just do:
df[col] = df[col].str.strip() # or .replace as above