Creating a list/tuple/dictionary

[Duncan Booth]

"Andrew Wilkinson" <ajw126NO at SPAMyork.ac.uk> wrote in 
news:3e1ae3b4$0$161$65c69314 at mercury.nildram.net:

> When the Python interpreter comes across a statement such as...
> 
> a = [1,2,3]
> 
> how does it go about creating that list?
> 
> Does it convert the code into something equivalent to
> a = []
> a.append(1)
> a.append(2)
> a.append(3)
> (although obviously it would call the C functions directly)
> 
> or is there a quicker method that it uses, if so is that accessible from
> ordinary python code?

The interactive interpreter is your friend:
>>> def f():
	a = [1, 2, 3]

	
>>> import dis
>>> dis.dis(f)
          0 SET_LINENO               1

          3 SET_LINENO               2
          6 LOAD_CONST               1 (1)
          9 LOAD_CONST               2 (2)
         12 LOAD_CONST               3 (3)
         15 BUILD_LIST               3
         18 STORE_FAST               0 (a)
         21 LOAD_CONST               0 (None)
         24 RETURN_VALUE        
>>> 
> 
> The same question applies to tuples and dictionaries, although I'm guess
> that the same answer will apply to all of them.
> 
Tuples use BUILD_TUPLE but are otherwise the same. Dictionaries are a bit 
messier.

The BUILD_LIST and BUILD_TUPLE bytecodes take a constant number of values 
from the stack. If you are building a list of known length then you can use 
these codes simply by writing out a list literal, otherwise you have to 
build the list up a bit at a time. There isn't any way to get a for loop to 
push values onto the stack and then use a BUILD_LIST with a variable to pop 
them all in one go.

-- 
Duncan Booth                                             duncan at rcp.co.uk
int month(char *p){return(124864/((p[0]+p[1]-p[2]&0x1f)+1)%12)["\5\x8\3"
"\6\7\xb\1\x9\xa\2\0\4"];} // Who said my code was obscure?