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pythonoperator-precedence

How is x += 1 <= y evaluated in Python?

How is x += 1 <= y evaluated in Python?

Intuitively I assume it is x += (1 <= 2) but it seems to be (x += 1) <= 2, however without any return value and x += 1 returning the value of x before the operation is executed. An explanation would be great!

Thanks!

Solution

  • No, the assignment operators have lower precedence than the comparison operators, so it is evaluated as x += (1 <= y).

    >>> x = 3
>>> y = 7
>>> x += 1 <= y
>>> x
4

    1 <= y is True, which has the value 1, and 1 is added to x.

    Python does not treat assignment operators like C. You cannot use them in mid-expression, so:

    >>> z = (x+=1) + (x+=1)
  File "<stdin>", line 1
    z = (x+=1) + (x+=1)
           ^
SyntaxError: invalid syntax