In most C-like languages that would be a syntax error. E.g. in C and C++ as a rule you tokenize "greedily", "1--2" would be tokenized as "1", "unary decrement operator", "2", which is illegal because you're trying to decerment an rvalue.

Python doesn't have "--", which allows the tokenizer to do something else.

▲ nyrikki 7 months ago | parent | next [-]

In C, that is really because Unary minus (negation) has precedence over binary operations.

    +a - b; // equivalent to (+a) - b, NOT +(a - b)
    -c + d; // equivalent to (-c) + d, NOT -(c + d)

https://en.cppreference.com/w/cpp/language/operator_arithmet...

    +-e; // equivalent to +(-e), the unary + is a no-op if “e” is a built-in type
     // because any possible promotion is performed during negation already

The same doesn't apply to, !! Which is applied as iterated binary operations (IIRC)

I am pretty sure the decriment operator came around well after that quirk was established.

	▲	seanhunter 7 months ago \| parent [-]
		Peter van der Linden’s book “Expert C Programming” (which is awesome btw) says that one of them (Kernighan, Richie or maybe Ken Thompson I forget) realised early on that the c compiler had the wrong operator precedence for bit twiddling and unary and boolean operators but “at that stage we had a few thousand lines of C code and thought it would be too disruptive to change it”

▲ j2kun 7 months ago | parent | prev [-]

Also worth noting that `1 - -2` works and produces 3 in C because the space breaks the operator.