How to pass values to Python?

Question

I would like to pass Mathematica variable values to Python. But I found that it was not that trivial. For example, a typical way would be:

session = StartExternalSession["Python"];
l = Range[10];
res = ExternalEvaluate[session, "
l=list(<*l*>)
print(l)
l
"];
DeleteObject[session];

The results are: a Python expression [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]:

res
{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

It seems to work. However, when I tried to define a Mathematica function with local variables:

f[l_] := Module[
  {session, res},
  session = StartExternalSession["Python"];
  res = ExternalEvaluate[session, "
l=list(<*l*>)
print(l)
l
"];
  DeleteObject[session];
  Return[res]
  ]

Clear[l]
f[Range[10]]

it didn't work. It looks like the "<* *>" notation doesn't accept local variables (in this case, l_List). I tried several patterns, but all my attempts failed in vain. After all, I needed to override the local variable l_LIst with another global variable l_List as follows.

l = Range[10];
f[Range[10]]

Did I miss something? Any suggestion would be welcome.

Answer 1

From the documentation (Scope > Command Otions > Arguments):

ExternalEvaluate[
 "Python",
 "lambda a, b: a+b" -> {1, 3}
 ]
(* 4 *)

So the easiest will be to define a function in your Python expression. Then you can pass the arguments as shown above, without any issues caused by localization and similar effects.

Answer 2

You can wrap python code into a function and use ExternalFunction. Pass values as args, forward args conversion is automatic.

session = StartExternalSession["Python"] ;

f = ExternalFunction[session, "def function(x): return ''.join(x)"] ;
x = {"a", "b", "c"} ;
f[x]
(* abc *)

f = ExternalFunction[session, "def function(x): return x**2"] ;
x = N[Range[10]] ;
f[x]
(* {1.`,4.`,9.`,16.`,25.`,36.`,49.`,64.`,81.`,100.`} *)

f = ExternalFunction[session, "import numpy\ndef function(x): return numpy.asarray(x)**2"] ;
x = N[Range[10]] ;
f[x]
Normal[x]
(* NumericArray[...] *)
(* {1.`,2.`,3.`,4.`,5.`,6.`,7.`,8.`,9.`,10.`} *)

DeleteObject[session];

Answer 3

From the help:

The command "cmd" may contain an inline template expression <expr>. This evaluates expr before "cmd" is sent to the external evaluator.

Now, the problem seems to com from this step. As long as the template contains local or global variables, this will work:

f[ll_] := Module[{session, res, arg},
  arg = ll;
  session = StartExternalSession["Python"];
  res = ExternalEvaluate[session, "
l=list(<*arg*>)
print(l)
"];
  DeleteObject[session];
  Return[res]]

f[Range[10]]

However, the template fill in does not work with function arguments:

f[ll_] := Module[{session, res, arg},
  session = StartExternalSession["Python"];
  res = ExternalEvaluate[session, "
l=list(<*ll*>)
print(l)
"];
  DeleteObject[session];
  Return[res]]

f[Range[10]]