how to apply a function to multiple columns in a pandas dataframe at one time
You can do df[['Col1', 'Col2', 'Col3']].applymap(format_number). Note, though that this will return new columns; it won't modify the existing DataFrame. If you want to put the values back in the original, you'll have to do df[['Col1', 'Col2', 'Col3']] = df[['Col1', 'Col2', 'Col3']].applymap(format_number).
You could use apply like this:
df.apply(lambda row: format_number(row), axis=1)You would need to specify the columns though in your format_number function:
def format_number(row): row['Col1'] = doSomething(row['Col1'] row['Col2'] = doSomething(row['Col2']) row['Col3'] = doSomething(row['Col3'])This is not as elegant as @BrenBarn's answer but it has an advantage that the dataframe is modified in place so you don't need to assign the columns back again