Why is my pandas dataframe turning into 'None' type?

Question

I can't see where is my problem, this code worked ok for a very simple example, but when I started working with my real data, I encountered problems.

I am basically extracting certain data from various csv files and trying to eventually combine them in a single dataframe.

The relevant piece of code is below:

wavenames = ['W1_', 'W2_', 'W3_']
logs=['log1','log2','log3','log4']

for w in wavenames:
    AllSynt = pd.DataFrame(index=range(6341), columns=['X']+logs)
    AllSynt['X']=z # an array extracted from elsewhere
    print AllSynt.head(3)
    for f in files:
        for l in logs:
            if (f.startswith('Synthetic_'+w)) & (f.endswith(l+'.csv')):
                df = pd.read_csv(path+f,delimiter=',')  
                AllSynt = pd.DataFrame(AllSynt)
                AllSynt = AllSynt.merge(df,how='left',on='X')
                AllSynt = AllSynt.rename(columns = {'Y':l}, inplace = True)
    print '\n', AllSynt.head(5)

but this gives me AttributeError: 'NoneType' object has no attribute 'head' (before making sure that the AllSynt is a pandas dataframe at the beginning of the loop, I got the same error (just saying it has no attribute 'merge'). Why is my AllSynt dataframe permanently turning into a None?

Answer 1

You would need to rewrite the line :

AllSynt = AllSynt.rename(columns={'Y':l}, inplace=True)

to simply the following:

AllSynt.rename(columns={'Y':l}, inplace=True)  # No assigning with inplace parameter
# (or)
AllSynt = AllSynt.rename(columns={'Y':l})       # assign without inplace parameter

When you specify inplace=True and want to see it's contents, it would return None as they merely mutate the DF instead of creating a new copy of it. Basically, you're assigning None to the result and hence it complains of the AttributeError as it isn't a pd.DataFrame object anymore to access it's .head() method.

Similar analogy could be observed by doing list.append(), list.sort() etc operations in pure python while assigning their results to a variable in the same line, which would also return None for the same reason as they operate inplace by default.

Edit: Added a close parenthesis.