CODEWITHSUNDEEP
pythonpython-2.7
0x016F2818
0x016F2818

Reputation: 168

What are the semantics of parentheses in this in python expression?

This python statement gives the expected value when I wrap each shift with parentheses, otherwise it gives 0 :

Coef = ((0xFF&resp[0])<<24) + ((0xFF&resp[1])<<16) + ((0xFF&resp[2])<<8) +(0xFF&resp[3])) #OK

Coef = (0xFF&resp[0])<<24 + (0xFF&resp[1])<<16 + (0xFF&resp[2])<<8 +0xFF&resp[3]) #NOK

From my point of view as a C developer, having in mind that both Coef and resp are unsigned ints this statements should be equivalent.

My guess is that Python is confused about the type of the var when the parenthesis are absent.

Upvotes: 0

Views: 103

Answers (1)

Ry-
Ry-

Reputation: 224904

Nothing to do with types; + has a higher precedence than <<. (This is also the case in C.) Your expression is parsed as:

((0xFF & resp[0])
    << (24 + (0xFF & resp[1]))
    << (16 + (0xFF & resp[2]))
    << (8 + 0xFF & resp[3]))

Also, if resp is a bytes (there’s a good chance it should be), you can just:

int.from_bytes(resp[:4], 'big')

If you have to use Python 2 for some reason, struct.unpack can do the same job:

Coef, = struct.unpack('>I', resp[:4])

Upvotes: 5

Related Questions