[Python] Python - How to check for missing values not represented by NaN?

I am looking for guidance on how to check for missing values in a DataFrame that are not the typical "NaN" or "np.nan" in Python. I have a dataset/DataFrame that has a string literal "?" representing missing data. How can I identify this string as a missing value?

When I run usual commands using Pandas like:

missing_values = df.isnull().sum()

print(missing_values[missing_values > 0])

Python doesn't pick up on these cells as missing and returns 0s for the sum of null values. It also doesn't return anything for printing missing values > 0.

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