Appending rows to empty DataFrame not working

Question

I have a piece of Python code that essentially reduces to the following:

import pandas as pd

months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
l = [1607.8, 1688.17, 1605.66, 1469.03, 1312.65, 1451.74, 1445.26, 1350.94, 1371.27, 1201.6, 1240.59, 1291.43]

# First, construct a empty DataFrame.
df = pd.DataFrame(columns = months, dtype=float)

# Then, keep adding rows to DataFrame given by list l.
df.append([l])

print df

Expected output is:

      Jan      Feb      Mar      Apr      May      Jun      Jul      Aug  \
0  1607.8  1688.17  1605.66  1469.03  1312.65  1451.74  1445.26  1350.94   

       Sep     Oct      Nov      Dec  
0  1371.27  1201.6  1240.59  1291.43  

However what I get is the following:

Empty DataFrame
Columns: [Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec]
Index: []

If I replace df.append([l]) with the following, things work fine for me.

df = pd.DataFrame([l], columns = months)
df.columns = months

What am I doing wrong?

Answer 1

You should use pd.DataFrame.loc to add a row given a list l:

df.loc[len(df.index)+1] = l

The method pd.DataFrame.append is used to append one dataframe, or other appendable object, to an existing dataframe:

DataFrame.append(other, ignore_index=False, verify_integrity=False, sort=None)

Append rows of other to the end of this frame

---

However, it is more efficient to form a list of lists and then call pd.DataFrame.append just once.

list_of_lists = []

for item in some_iterable:
    l = foo(item)
    list_of_lists.append(l)

df = df.append(pd.DataFrame(list_of_lists, columns=df.columns))

The reason for this is list.append is cheaper than pd.DataFrame.loc assignment.

Answer 2

pd.append() does not happen in place. You need to save the output in order to use it.