[mypyc] fix: inappropriate Nones in f-strings (#19846)

if a variable is Final but the value is not yet known at compile-time,
and that variable is used as an input to an f-string, the f-string will
incorrectly contain "None"

Fixes [mypyc#1140](mypyc/mypyc#1140)

Author: BobTheBuidler

mypyc/irbuild/specialize.py

@@ -719,7 +719,9 @@ def get_literal_str(expr: Expression) -> str | None:
             if isinstance(expr, StrExpr):
                 return expr.value
             elif isinstance(expr, RefExpr) and isinstance(expr.node, Var) and expr.node.is_final:
-                return str(expr.node.final_value)
+                final_value = expr.node.final_value
+                if final_value is not None:
+                    return str(final_value)
             return None
 
         for i in range(len(exprs) - 1):

mypyc/test-data/run-strings.test

@@ -412,9 +412,16 @@ def test_basics() -> None:
 [case testFStrings]
 import decimal
 from datetime import datetime
+from typing import Final
 
 var = 'mypyc'
 num = 20
+final_known_at_compile_time: Final = 'hello'
+
+def final_value_setter() -> str:
+    return 'goodbye'
+
+final_unknown_at_compile_time: Final = final_value_setter()
 
 def test_fstring_basics() -> None:
     assert f'Hello {var}, this is a test' == "Hello mypyc, this is a test"
@@ -451,6 +458,8 @@ def test_fstring_basics() -> None:
     inf_num = float('inf')
     assert f'{nan_num}, {inf_num}' == 'nan, inf'
 
+    assert f'{final_known_at_compile_time} {final_unknown_at_compile_time}' == 'hello goodbye'
+
 # F-strings would be translated into ''.join[string literals, format method call, ...] in mypy AST.
 # Currently we are using a str.join specializer for f-string speed up. We might not cover all cases
 # and the rest ones should fall back to a normal str.join method call.