单行 if/else unexpected SyntaxError

Question

Python one-line if/else statements should function without raising a SyntaxError, but with some keywords they do.

The expected results of pass if arg else pass should be similar to print('') if arg else print('') . While they appear similar, one results in a syntax error and the other does not. Why does the interpreter permit some keywords to be used this way and not others?

def foo(arg):
    #raise BaseExecption if arg else raise BaseException
    #pass if arg else pass
    print('') if arg else print('')
    
foo(True)

Note: The commented out lines of code will generate a SyntaxError.

Answer 1

pass is a statement (specifically a "simple statement"); it isn't an expression. That's a problem because conditional expressions ("ternaries") only work on expressions. Here's their entry in the Python grammar:

disjunction 'if' disjunction 'else' expression

All parts there are expressions besides the keywords.

This isn't really a problem through since you're abusing conditional expressions in the first place. Instead, use a full if -statement.