Closes #9418: Fix missing bracket for empty Callable annotations.

[thjungers]

Subject: Fix missing bracket for empty Callable annotations. This solves issue #9418

Feature or Bugfix

• Bugfix

Purpose

• When rendering a Callable type annotation, the output is missing a bracket if the Callable requires no parameters (the first element is empty): Callable[[], None] is rendered as Callable[], None].

Detail

When unparsing a parsed annotation containing an ast.List object, the following pseudo-code is run:

result.append('[')
for elem in node.elts:
    result.append(elem)
    result.append(', ')
result.pop()
result.append(']')

The last element (the extra comma) is popped. However, if the ast.List object is empty, the for-loop is skipped and the popped element is the opening [.
An if-condition was added to check whether node.elts is empty before popping the last element of result.

Relates

• Issue Type annotation: missing bracket for Callable with no arguments #9418

[tk0miya]

Merge this manually into the 4.x branch now.
Thank you for your contribution!