pandas.DataFrame.replace
df = df.replace('无', np.nan)
pandas.DataFrame.applymap
df = df.applymap(lambda x: str(x).strip())
pandas.DataFrame.rename
*
df.columns = ['a', 'b', 'c']
# using keyword arguments to clarify your intent:
df = df.rename(columns={'A': 'a', 'B': 'b', 'C': 'c'})
# axis-style:
df = df.rename(str.lower, axis='columns')