Mapping str() over columns in a dataframe returns "TypeError: object of type 'map' has no len()"

Im writing a program that takes in an excel-file, reorders the columns and discards the scraps. Everything works fine except for when i tried to rewrite a function that fixes all of the columns sio it wasent a complete mess. The result was the following:

def fixColsT(df1):
    null_check = np.eye(len(df1.iloc[:,1]),12) #create matrix
    null_check = map(np.isfinite, df1.iloc[:,1:12]) #set matrix-values.
    null_check = map(np.invert, df1.iloc[:,1:12]) #Invert for error-checking (not_added)
    df1.iloc[:,3:4] = map(str,df1.iloc[:,2:4]) #The problem.
    df1.iloc[:,3] = map(lambda entry: re.sub(r'[^0-9]', '', entry), df1.iloc[:,3])
    df1.iloc[:,4] = map(lambda x:x [len(x)-1], df1.iloc[:,4])
    df1.iloc[:,4] = map(lambda x: re.sub(r'[^A-Ö]', '', x), df1.iloc[:,4])
    df1.iloc[:,2] = (df1.iloc[:,2] - df1.iloc[:,4]) - df1.iloc[:,3] 
    df1.iloc[:,11] = map(fixnrRooms, df1.iloc[:,11])
    df1.iloc[:,12] = map(fixKitchen, df1.iloc[:,12])
    df1.columns = COLUMN_NAMES
    errorcheck = null_check.iloc[:,2]
    for i in CHECKS:
        errorcheck = errorcheck + null_check[:,CHECKS[i]]
    errors = np.where(errorcheck)[0]
    print(df1.iloc[4,:])
    return df1, errors

When i run this, i get the following error:

Traceback (most recent call last):
  File "[...]\main.py", line 403, in <module> 
    main()
  File "[...]\main.py", line 391, in main     
    df, errors=fixColsT(dfInput)
  File "[...]\main.py", line 118, in fixColsT 
    df1.iloc[:,3:4] = map(str,df1.iloc[:,2:4])
  File "[...]\Python\Python310\lib\site-packages\pandas\core\indexing.py", line 818, in __setitem__
    iloc._setitem_with_indexer(indexer, value, self.name)
  File "[...]\Python\Python310\lib\site-packages\pandas\core\indexing.py", line 1795, in _setitem_with_indexer
    self._setitem_with_indexer_split_path(indexer, value, name)
  File "[...]\Python\Python310\lib\site-packages\pandas\core\indexing.py", line 1836, in _setitem_with_indexer_split_path
    elif len(ilocs) == 1 and lplane_indexer == len(value) and not is_scalar(pi):
TypeError: object of type 'map' has no len()

I honestly have no idea how to proceed (keep in mind that im reasonably new to python in general) as i need to somehow convert the entries in these columns to strings for the rest of the code to work. (The previous solution iterated over every row of the entire dataframe and casted every entry manually which is fine for very small files but as it can be expected to get files of around 60000 rows, this is not reasonable).
How would one get around this without iterating over every entry or over every column individually (unless theat is absolutely neccessary).

Looking at that line:

df1.iloc[:,3:4] = map(str,df1.iloc[:,2:4])

The left hand side is a (column) slice slice of the dataframe, and the right-hand side is a map apply a function on a dataframe.
Two comments:

• map does not work well on dataframe, it's ambiguous if you want to apply a function along rows, columns or on each entry (I think that's what you want here). Use DataFrame.apply or DataFrame.applymap instead, map for series.
• The return of a map is a lazy iterator, but the = on the lhs slice would like to now the length (even shape) of the what is being copied, this is not trivially readable from a map (you need to exhaust it and store all its values).

All in all, try using:

    df1.iloc[:,3] = df1.iloc[:,2:4].agg(''.join, axis=1)
    df1.iloc[:,3] = df1.iloc[:,3].str.replace(r'[^0-9]', '')
    df1.iloc[:,4] = df1.iloc[:,4].str.slice(-1)
    df1.iloc[:,4] = df1.iloc[:,4].str.replace(r'[^A-Ö]', '')
    df1.iloc[:,2] = (df1.iloc[:,2] - df1.iloc[:,4]) - df1.iloc[:,3] 
    df1.iloc[:,11] = df1.iloc[:,11].map(fixnrRooms)
    df1.iloc[:,12] = df1.iloc[:,12].map(fixKitchen)