python - Add leading zeroes only if it begin with digit in pandas dataframe

Question

I have a data frame column which have special characters and numbers. I want to have leading zeroes only if it begin with a digit. I need a total of 3 digits. I tried the following code:

df['input'] = df['input'].str.zfill(3)

Input:

column
1
$500
333
2
(8
?8
5
1
444

Answer 1

you can get what you want by:

df_merge['output'] = df_merge['input'].apply(lambda x : f'{x:0>3}' if x[0] in '0123456789' else x)

Verification

Assume the followin example to simulate the case you gave:

import pandas as pd
df_merge = pd.DataFrame({'input':['1','$500','333','2','(8','?8']})

print(df_merge['input'])

0       1
1    $500
2     333
3       2
4      (8
5      ?8
Name: input, dtype: object

now apply:

df_merge['output'] = df_merge['input'].apply(lambda x : f'{x:0>3}' if x[0] in '0123456789' else x)

print(df_merge['output'])

0     001
1    $500
2     333
3     002
4      (8
5      ?8
Name: output, dtype: object

Good Luck