This issue was already discussed here, but the answer is unclear yet. I use very simple Python function for converting a string list to upper case

def ListToUpper(wrd_lst):
  for i in range(len(wrd_lst)):
    string_list[i] =string_list[i].upper()
  return string_list

Code ListToUpper(['a','b']) works in Python as needed. But Mathematica displays an error when one calls (ss -- session)

ExternalEvaluate[ss, <| "Command" -> "ListToUpper","Arguments" -> {"['a', 'b']"} |>]

It's strange, as the argument ['a', 'b'] is the same for both calls. In the case, when argument is a scalar (or string), all works properly

  • $\begingroup$ What error does it display? Please add it to the question. $\endgroup$
    – flinty
    Nov 24, 2021 at 11:27
  • $\begingroup$ "This issue was already discussed here, but the answer is unclear yet." Which post are you refering to? Can you add the link to the question? $\endgroup$
    – xzczd
    Nov 24, 2021 at 11:56

2 Answers 2


By taking advantage of Python's list comprehension capability, this method does not need to convert the tuple wrd_list to a list

ss = StartExternalSession["Python"]

listToUpper = ExternalFunction[ss, "def ListToUpper(wrd_lst):
     return [i.upper() for i in wrd_lst]"];

listToUpper[{"a", "b"}]

(*   {A, B}  *)
  • $\begingroup$ For me, it is desirable to use an external Python function, because I apply more comprehensive processing of a list (related to language stemming). I'm not sure that ExternalFunction will support external Python function in *.py file :(( $\endgroup$
    – Konstantin
    Nov 24, 2021 at 12:31
  • 1
    $\begingroup$ @Konstantin why wouldn't it support that? Can you import the file that exports the function you want? $\endgroup$
    – Jason B.
    Nov 24, 2021 at 13:12
  • $\begingroup$ The LouisB's code works fine for my problem of stemming. But executing a Python file with ExternalEvaluate is more preferable for some reasons. For example, my Python file can include a number of functions, so they can be called by ExternalEvaluate[ss, <| "Command" -> "coms", "Arguments" -> {args} |>] without definition of new ExternalFunction for each Python function $\endgroup$
    – Konstantin
    Nov 24, 2021 at 17:41
  • $\begingroup$ Thank you to all! The problem is solved by calling: ExternalEvaluate[ss, File["PythonFile"]]; ExternalEvaluate[ss, <| "Command" -> "com", "Arguments" -> {{arg1, arg2, ..., argn}} |>] $\endgroup$
    – Konstantin
    Nov 24, 2021 at 17:52

This works fine:

ss = StartExternalSession["Python"];
ListToUpper = ExternalFunction[ss, "def ListToUpper(wrd_lst):
  for i in range(len(wrd_lst)):
    string_list[i] = string_list[i].upper()
  return string_list"];

ListToUpper[{"a", "b"}]

(* {"A", "B"} *)

Note that I used list(wrd_lst) as wrd_lst is passed as a tuple and tuples are immutable later on at the point string_list[i] = ... so you must convert to a mutable list first.


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