python pandas: how to add a value to specific rows

Question

I have a csv where one column is a six or seven digit ID number. I want to add the number 0 to the beginning of all the IDs that have six digits. How is this possible?

for example

id

203434
405679
1049294
139294
1037912

I would like it to be

id

0203434
0405679
1049294
0139294
1037912

any advice appreciated.

Answer 1

You will need to convert the series dtype to string first, because integral values do not start with leading zeroes. Once you have a string you can use the Series.str.zfill method to add leading zeroes.

Try the following:

df["id"] = df["id"].astype("string").str.zfill(7)

The 7 will ensure that your id values are zero-padded on the left up to 7 characters.