Why is 'True == not False' a syntax error in Python?

It has to do with operator precedence in Python (the interpreter thinks you're comparing True to not, since == has a higher precedence than not). You need some parentheses to clarify the order of operations:

True == (not False)

In general, you can't use not on the right side of a comparison without parentheses. However, I can't think of a situation in which you'd ever need to use a not on the right side of a comparison.

It's just a matter of operator precedence. Try:

>>> True == (not False)
True

Have a look in this table of operator precedences, you'll find that == binds tigher than not, and thus True == not False is parsed as (True == not) False which is clearly an error.

Answers claiming that the reason for True == not False constituting a syntax error had to do with operator precedence are mistaken. If that were the case, the expression 2 ** - 1 would yield a syntax error as well, which of course it doesn't. Precedence never causes an operator to be drawn in in place of an operand.

The true reason for True == not False being a syntax error is that there exists no syntax rule that would produce a comparison therefrom, since

comparison ::= or_expr (comp_operator or_expr)*

- i. e. after the comp_operator == an or_expr must follow, which includes an xor_expr, an and_expr, a shift_expr, an a_expr, an m_expr, an u_expr, a power…, but no not_test.

By comparison, the precedence-wise similar construct 2 ** - 1 in accordance with the power rule

power ::= (await_expr | primary) ["**" u_expr]

has u_expr following the power operator **, thus allowing - x on the right hand side.