[Python] Python Unit Test Side Effect Returns Function Instead of Value

I am working through creating unit tests for a Python application that handles data via Pandas DataFrames. One section of this application formats data in various DataFrame columns. I am looking to mock the Series.apply function to format column values into strings.

The line I am looking to mock is:

source_table_df["col1"] = source_table_df["col1"].apply(self._format_col)


I am mocking this with the following code:

with patch("pandas.core.series.Series.apply") as mock_apply:
mock_apply.side_effect = (lambda val: str(val))


For my unit test, I have a table that looks like so:

col1
===
1
1
1


And I am looking for this to be converted to:

col1
===
"1"
"1"
"1"


However, I am recieving the following outcome:

col1
===
<function MyClass._format_col at 0x000001AA25FD0A40>
<function MyClass._format_col at 0x000001AA25FD0A40>
<function MyClass._format_col at 0x000001AA25FD0A40>


How can I properly mock the conversion of the column values? Thank you for your time.

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