CPython does constant folding but mypyc doesn't. This can result in performance reduction in compiled code, so it's important to have this working, at least in common cases.

For example, consider this code:

C: Final = 5
x = [4 * 6 - 3, 1 + C]

This should generate the same code as this:

C: Final = 5
x = [21, 6]

Based on some experimentation, at least these things are supported by CPython:

• Integer arithmetic
• Bitwise operations
• Float arithmetic
• String operations
  • +
  • *
• Bytes operations
  • +
  • *
• Complex + int OR int + Complex
• Tuple construction
• Tuple indexing (int index)
• String indexing (int index)
• Tuple concatenation

#726 and #619 are related.

For extra credit, we can also detect local variables that have constant values:

def f() -> None:
    x = 'foo'
    y = x + 'bar'   # Simplify to y = 'foobar'
    ...