On Mon, 26 Mar 2007 15:53:56 -0700, sturlamolden wrote:
> Python is a very high-level language. That means there are certain
> things that put constraint on the attained speed. Most importantly:
> keep the number of interpreter evals as scarce as possible. If you
> make a for loop, the interpreter may evaluate the lines within the
> loop several times. Languages like C, C++ and Java really teach you
> bad habits when it comes to an interpreted language like Python. In
> these languages, loops are almost free. In Python, they may be very
> expensive as the interpreter is invoked multiple times.
Jeez-Louise, this is why my blood boils when I read ignora^H^H^H^H
misguided people referring to Python as "interpreted": it leads other
people to imagine that Python is interpreting lines over and over and over
again.
Is the interpreter invoked multiple times in a for loop? Let's find out.
Here is a function with a for loop:
def looper():
x = 1
for i in range(1000000):
print x
x += i
return x
Let's look at the compiled code:
>>> dis.dis(looper)
2 0 LOAD_CONST 1 (1)
3 STORE_FAST 0 (x)
3 6 SETUP_LOOP 35 (to 44)
9 LOAD_GLOBAL 0 (range)
12 LOAD_CONST 2 (1000000)
15 CALL_FUNCTION 1
18 GET_ITER
>> 19 FOR_ITER 21 (to 43)
22 STORE_FAST 1 (i)
4 25 LOAD_FAST 0 (x)
28 PRINT_ITEM
29 PRINT_NEWLINE
5 30 LOAD_FAST 0 (x)
33 LOAD_FAST 1 (i)
36 INPLACE_ADD
37 STORE_FAST 0 (x)
40 JUMP_ABSOLUTE 19
>> 43 POP_BLOCK
6 >> 44 LOAD_FAST 0 (x)
47 RETURN_VALUE
Certainly Python isn't interpreting the lines in the for loop one million
times. It interprets them once, compiles them once, and executes them one
million times.
> But if you can
> 'vectorize' the loop statement into a one-liner (or a few lines) using
> slicing, list comprehensions or functional programming (e.g. the
> lambda, map and filter intrinsics), the overhead will be very small.
This is generally good advice because it moves the loop from
moderately fast Python code to very fast C code, not because Python is
interpreting each line over and over again.
--
Steven.
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