Schema changes after running Python script

I have a simple python script that I run after an initial dataset is pulled into my flow as it takes the name of each column and strips out blank spaces and lowers all case. I have seen in some situations that the initial dataset is a string but then after running the python script it changes it to a bigint. For instance data that has preceding 0's like 0003928. How do I prevent this from occurring?

I use the default code except for the 1 line I highlighted below. Thanks for any help you may be able to provide!

# -*- coding: utf-8 -*-
import dataiku
import pandas as pd, numpy as np
from dataiku import pandasutils as pdu

# Read recipe inputs
pmoexportSetProblem = dataiku.Dataset("PMOExportSetProblem")
pmoexportSetProblem_df = pmoexportSetProblem.get_dataframe()

# Compute recipe outputs from inputs
# TODO: Replace this part by your actual code that computes the output, as a Pandas dataframe
# NB: DSS also supports other kinds of APIs for reading and writing data. Please see doc.

sn_problem_df = pmoexportSetProblem_df # For this sample code, simply copy input to output
sn_problem_df.columns = sn_problem_df.columns.str.strip().str.lower().str.replace(' ', '_').str.replace('(', '').str.replace(')', '').str.replace('.', '_').str.replace('/', '_').str.replace('&', 'n').str.replace("'", '')

# Write recipe outputs
sn_problem = dataiku.Dataset("sn_problem")
sn_problem.write_with_schema(sn_problem_df)