Because that's how the grammar is defined: https://docs.python.org/3.8/reference/grammar.html https://docs.python.org/3.8/reference/grammar.html The RHS of th

Because that's how the grammar is defined: https://docs.python.org/3.8/reference/grammar.html https://docs.python.org/3.8/reference/grammar.html The RHS of the == (comp_op) has to be an expr. You can force "not x" to be an expr by parenthesizing it: x == (not x)