This small program

mac2011% cat bug.py
print(__name__ == "__main__")
#if True:
if __name__ == "__main__":
     print("main")
else:
     print("not main")

works well in Terminal :

mac2011% python bug.py
True
main

mac2011% python --version
Python 3.6.9 :: Anaconda, Inc.

but it does not work in Emacs python mode :

mac2011% Emacs -Q bug.py

(version)
"GNU Emacs 28.0.50 (build 1, x86_64-apple-darwin15.6.0, NS appkit-1404.47 
Version 10.11.6 (Build 15G22010))
  of 2020-01-09"

Files opens in (Python ElDoc) mode

Python menu

Start interpreter (C-c C-p)

Eval buffer (C-c C-c)

Python 3.6.9 |Anaconda, Inc.| (default,     Jul 30 2019, 13:42:17)
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)] on darwin
Type "help", "copyright", "credits" or     "license" for more information.
 >>> python.el: native completion setup loaded
 >>> Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "/Users/alba/Documents/projets/plot time series/bug.py", line 5
     else:
        ^
SyntaxError: invalid syntax
 >>>

Replacing 'if __name__ == "__main__":' by 'if True:', there is no more error.

Replacing "__main__" by "__main_", there is no more error.

Same with Python 3.7.6.

'print("main") if __name__ == '__main__' else  print("not main")' works.

Best regards.

-- 
Pierre ALBARÈDE