JJP JJP - 2 months ago 10
Python Question

How about 'empty if statement' in python

It's very common in C: hacking 'empty if statement' like this:

if(mostlyhappencondition)
;#empty statement
else{
dosomething;
}


It's working in Python? I mean, can we improve the app's performance by using that? I'd also like to know why.

Answer

There is a performance improvement if there isn't an else case in the "if", since the bytecodes don't pass execution into the "if" case.

Here's some functions and the output of dis.dis(foo)

The following sample app:

def foo(x):
    if x:
        pass
    else:
        return x+2

Disassembles to:

5           0 LOAD_FAST                0 (x)
            3 POP_JUMP_IF_FALSE        9

6           6 JUMP_FORWARD             8 (to 17)

8     >>    9 LOAD_FAST                0 (x)
           12 LOAD_CONST               1 (2)
           15 BINARY_ADD          
           16 RETURN_VALUE        
      >>   17 LOAD_CONST               0 (None)
           20 RETURN_VALUE        

The following

def foo(x):
    if not x:
        return x+2

Disassembles to:

11           0 LOAD_FAST                0 (x)
             3 POP_JUMP_IF_TRUE        14

12           6 LOAD_FAST                0 (x)
             9 LOAD_CONST               1 (2)
            12 BINARY_ADD          
            13 RETURN_VALUE        
       >>   14 LOAD_CONST               0 (None)