Can you please explain what does the syntactic parse tree look like for a chained comparison?
As far as I understand in most of languages it constructs nodes based on operator associativity so in
a < b < c
a < b and b < c
The comparison grammar is not that interesting here, it simply lets you append multiple comparators to an operator:
comparison ::= or_expr ( comp_operator or_expr )* comp_operator ::= "<" | ">" | "==" | ">=" | "<=" | "!=" | "is" ["not"] | ["not"] "in"
So lets ask the Python parser directly, by using the
ast module (which simply asks the Python compiler itself to only return the Abstract Syntax Tree):
>>> import ast >>> ast.dump(ast.parse('a > b > c', mode='eval')) "Expression(body=Compare(left=Name(id='a', ctx=Load()), ops=[Gt(), Gt()], comparators=[Name(id='b', ctx=Load()), Name(id='c', ctx=Load())]))"
So there is just a single
Compare node, with multiple operators and comparators:
Compare( left=Name(id='a'), ops=[Gt(), Gt()], comparators=[Name(id='b'), Name(id='c')])
(I omitted the
This lets the interpreter evaluate comparators as needed (e.g. if
a < b is false, the remaining comparators don't have to be considered).
The resulting bytecode uses conditional jumps to skip remaining comparisons:
>>> import dis >>> dis.dis(compile('a > b > c', '', 'eval')) 1 0 LOAD_NAME 0 (a) 2 LOAD_NAME 1 (b) 4 DUP_TOP 6 ROT_THREE 8 COMPARE_OP 4 (>) 10 JUMP_IF_FALSE_OR_POP 18 12 LOAD_NAME 2 (c) 14 COMPARE_OP 4 (>) 16 RETURN_VALUE >> 18 ROT_TWO 20 POP_TOP 22 RETURN_VALUE