为什么Python显示错误：无法分配给运营商？

Question

from datetime import datetime
now = datetime.now()
q = int(now.strftime('%I'))
w = int(now.strftime('%M'))

def correct_time(delta):
    if w < (120 - delta):
        Q = q + 1 and W = w + (delta - 60)
    else:
        Q = q + 2 and W = w + (delta - 120)
    return Q, W

correct_time(45)

#gives error - SyntaxError: can't assign to operator

In the above code I was trying to assign new values of q and w to Q and W respectively. I get no error when I only try to assign new value to either Q or W (not to both in the same line) but when I use 'and'(as in the above case) or ',' I get such an error.

I searched the internet for 'can't assign to operator' error but found that they were not relevant to my case. I would like to know why the above code shows error when it follows the rules of assignment.

Answer 1

We all know that only expression and expression is valid. So the point here is that in Python, assignment is just a statement , not expression . In C , a = 3 is assignment expression and a = 3 && b = 4 is valid. But, this is not valid in Python.

And Python will have its own assignment expression in 3.8: PEP 572 -- Assignment Expressions | Python.org

In Python3.8, you will be able to do such thing:

# This block code only works with Python3.8+
from datetime import datetime
now = datetime.now()
q = int(now.strftime('%I'))
w = int(now.strftime('%M'))

def correct_time(delta):
    if w < (120 - delta):
        (Q := q + 1) and (W := w + (delta - 60))
    else:
        (Q := q + 2) and (W := w + (delta - 120))
    return Q, W

correct_time(45)

But, it is not necessary, since you can just do such thing in Python:

a, b = 1, 2

which equals to

a = 1
b = 2

Answer 2

Your code should be:

def correct_time(delta):
    if w < (120 - delta):
        Q = q + 1
        W = w + (delta - 60)
    else:
        Q = q + 2
        W = w + (delta - 120)
    return Q, W

In an expression like a = 1 and b = 2 , Python would first try to assign the value 2 to a and b , which is impossible and produces the error SyntaxError: can't assign to operator . You would get something similar with a = 1 = 2 : here, trying to assign 2 to 1 would produce SyntaxError: can't assign to literal .

Answer 3

What you're looking for is the following syntax:

def correct_time(delta):
    if w < (120 - delta):
        Q, W = q + 1, w + (delta - 60)
    else:
        Q, W = q + 2, w + (delta - 120)
    return Q, W

In your case it doesn't work because right hand should be expression but it's expression and statement (due to assignment).