I have a variable df1 but when I try adding a new column, it gives a syntax error?

Question

I have some code:

import pandas
df1 = pandas.read_csv("index_data.csv")
difference = ""
df2 = df1.loc[0:434, "Close":"Close"]
def next_column():
    for row_index in range(0, 435):
        if row_index % 2 == 0:
            difference = "0"
        else:
            difference = str((row_index)

df1["Difference between close"] = df1[difference]

I am not sure why but it is giving me the error:

  File "<ipython-input-16-8820765d3a96>", line 10
df1["Difference between close"] = df1[difference]
^
SyntaxError: invalid syntax