How to replace empty cells with 0 and change strings to integers where possible in a pandas dataframe?

Question

I have a dataframe with a 3000+ columns. Many cells in the dataframe are empty strings (' '). Also, I have a lot of numerical values that are are strings but should actually be integers. I wrote two functions to fill all the empty cells with a 0 and where possible change the value to an integer, but when I run them nothing changes to my dataframe. The functions:

def recode_empty_cells(dataframe, list_of_columns):

    for column in list_of_columns:
        dataframe[column].replace(r'\s+', np.nan, regex=True)
        dataframe[column].fillna(0)

    return dataframe

def change_string_to_int(dataframe, list_of_columns):

    dataframe = recode_empty_cells(dataframe, list_of_columns)

    for column in list_of_columns:
        try:
            dataframe[column] = dataframe[column].astype(int)
        except ValueError:
            pass

    return dataframe

Note: I'm using a try/except statement because some columns contain text in some form. Thanks in advance for your help.

Edit:

Thanks to your help I got the first part working. All the empty cells have 0s now. This is my code at this moment:

def recode_empty_cells(dataframe, list_of_columns):

    for column in list_of_columns:
        dataframe[column] = dataframe[column].replace(r'\s+', 0, regex=True)

    return dataframe

def change_string_to_int(dataframe, list_of_columns):

    dataframe = recode_empty_cells(dataframe, list_of_columns)

    for column in list_of_columns:
        try:
            dataframe[column] = dataframe[column].astype(int)
        except ValueError:
            pass

    return dataframe

However, this gives me the following error: OverflowError: Python int too large to convert to C long

Answer 1

consider the df

df = pd.DataFrame(dict(A=['2', 'hello'], B=['', '3']))
df

---

apply

def convert_fill(df):
    return df.stack().apply(pd.to_numeric, errors='ignore').fillna(0).unstack()

convert_fill(df)