How to create an 'empty if statement' in python

Question

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 1

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) 

Answer 2

I can only guess you're looking for the pass statement, sometimes needed to create an empty code block to prevent a syntax error.

if mostlyhappencondition:     pass else:     do_something() 

It would be much more usual to just do this, which is logically equivalent:

if not mostlyhappencondition:     do_something() 

There are no significant performance gains to be found here.