I have Tried this , but not working and giving error .
the error is: ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), #a.any() or a.all().
df = pd.DataFrame(np.random.randn(5,5))
df[df > 0.9] = pd.np.nan
df.columns = ['a','b','c','d','e']
a=df.a
for column in df.columns:
if df.a.isnull()=='true':
column=np.random.randn(7,11)enter code here