elif与else if; 是一个比另一个快，看着python程序集

Question

Are these two code snippets exactly the same (like they would be in c++) or will they both produce slightly different running times?

first)

x = 'hello joe'

if x == 'hello':
  print('nope')
elif x == 'hello joe':
  print(x)

second)

x = 'hello jo'

if x == 'hello':
  print('nope')
else:
  if x == 'hello joe':
    print(x)

I wanted to find out myself but I am not sure how I might go about watching this code run in its assembly form in real time. Which brings me to my second question: how might I see the compiled assembly instructions that are made when I compile a Python program?

Answer 1

First, let's put your code(s) in a function

def func():               # line 1
    x = 'hello joe'       # line 2

    if x == 'hello':      # line 4
      print('nope')       # line 5
    else:                 # line 6
     if x == 'hello joe': # line 7
      print(x)            # line 8

now disassemble with that (using CPython 3.4):

import dis
dis.dis(func)

we get:

  2           0 LOAD_CONST               1 ('hello joe')
              3 STORE_FAST               0 (x)

  4           6 LOAD_FAST                0 (x)
              9 LOAD_CONST               2 ('hello')
             12 COMPARE_OP               2 (==)
             15 POP_JUMP_IF_FALSE       31

  5          18 LOAD_GLOBAL              0 (print)
             21 LOAD_CONST               3 ('nope')
             24 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             27 POP_TOP
             28 JUMP_FORWARD            25 (to 56)

  7     >>   31 LOAD_FAST                0 (x)
             34 LOAD_CONST               1 ('hello joe')
             37 COMPARE_OP               2 (==)
             40 POP_JUMP_IF_FALSE       56

  8          43 LOAD_GLOBAL              0 (print)
             46 LOAD_FAST                0 (x)
             49 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             52 POP_TOP
             53 JUMP_FORWARD             0 (to 56)
        >>   56 LOAD_CONST               0 (None)
             59 RETURN_VALUE

now change to elif:

  2           0 LOAD_CONST               1 ('hello joe')
              3 STORE_FAST               0 (x)

  4           6 LOAD_FAST                0 (x)
              9 LOAD_CONST               2 ('hello')
             12 COMPARE_OP               2 (==)
             15 POP_JUMP_IF_FALSE       31

  5          18 LOAD_GLOBAL              0 (print)
             21 LOAD_CONST               3 ('nope')
             24 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             27 POP_TOP
             28 JUMP_FORWARD            25 (to 56)

  6     >>   31 LOAD_FAST                0 (x)
             34 LOAD_CONST               1 ('hello joe')
             37 COMPARE_OP               2 (==)
             40 POP_JUMP_IF_FALSE       56

  7          43 LOAD_GLOBAL              0 (print)
             46 LOAD_FAST                0 (x)
             49 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             52 POP_TOP
             53 JUMP_FORWARD             0 (to 56)
        >>   56 LOAD_CONST               0 (None)
             59 RETURN_VALUE

The only differences are the line numbers.

    else:                 # line 6
     if x == 'hello joe': # line 7

becomes (and shifts the rest as well)

    elif x == 'hello joe': # line 6

There are as many instructions in both versions. The else and if keywords in that case seem to have been converted exactly the same way as elif . Not guaranteed in all implementations. Personally I'd stick to the shortest code ( elif ) because it's more "meaningful" and if a code should be faster, it would probably be that one.