CODEWITHSUNDEEP
pythonappenddataframe
emax
emax

Reputation: 7235

Append to a list does not work

I have to add a column called 'sessions' to a dataframe called merged1. The column sessions is update in the loop and it is the list y. However the following operations don't work:

y.append(x * len(data))
merged1['sessions'] = y

Here the code

for i in users:
    merged1 = pd.DataFrame()
    name = "%s" %i
    y = list()
    for file in glob.glob("*.csv"):
        if os.path.isfile(file):    # make sure it's a file, not a directory entry
            if name in file:   # open file
                data = pd.read_csv(file)
                data = data.loc[[k for j, k in enumerate(data.index) if j % 10 == 0]]
                data.lat = np.round(data.lat, 6)
                merged1 = pd.concat([merged1,data], ignore_index=True)
                x = re.findall(r'(?<=_session)\d+', file)
                y.append(x * len(data))
    merged1['sessions'] = y
    if len(merged1) > 0:
        merged1 = merged1[merged1.lat > 45]
        merged1.to_csv(string,index=False)

Upvotes: 0

Views: 84

Answers (1)

Anand S Kumar
Anand S Kumar

Reputation: 90899

When you do - 

y.append(x * len(data))

you are actually appending lists of size len(data) * len(x) into y so y becomes a list of lists.

Hence when you do - merged1['sessions'] = y - and the size of y differs from the size of merged1 , it causes issues.

If you are sure that x = re.findall(r'(?<=_session)\d+', file) would always return only 1 element , then you can use -

y.extend(x * len(data))

instead of .append() . .extend() extends the list with the elements from the iterable that is passed to it.

Upvotes: 2

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