Converting Categorical Variables to Numerical Variables by using def

I want to create a function that converts categorical variables to numerical variables. After then, I want to add one more thing, when I typed the dataframe in the function at the beginning, it has to replaces unique names with numeric values. But I have an issue while I am trying. I tried something like that:

def x(data):
 for col in data:
    if col.dtypes() == object:
        col.replace(list(df[col].unique()),
                    list(np.arange(data[col].nunique()), 
                    inplace = True)

Where am I wrong?

Thanks in advance for your opinions.