list displays

Sat Jan 8 17:41:41 EST 2011

On 2011年1月08日 22:57:45 +0100, Olive wrote:
> I am a newbie to python. Python supports what I thinks it is called list
> display, for example:
>> [i for i in range(10)]
> [i for i in range(10) if i<6]

This is called a list comprehension, not list display.
> Does anyone know a good documentation for this. I have read the language
> reference but it is confusing.

A list comprehension is syntactic sugar for a for loop. If you start with 
code looking like this:
storage = []
for i in range(10):
 if i < 6:
 storage.append(i)
you can re-write this as a list comprehension:
storage = [i for i in range(10) if i < 6]
The source doesn't have to be range, it can be any sequence or iterator:
lengths = [len(obj) for obj in my_list_of_objects]
# like map(len, my_list_of_objects)
If you are mathematically inclined, you might also like this analogy: the 
syntax for a list comprehension is similar to that of sets in mathematics.
[f(x) for x in D]
is similar to: 
{ f(x) ∀ x ∈ D }
("the set of f(x) for all x element of D")
Don't waste your time with list comprehensions that just walk over the 
source, doing nothing. For example:
[i for i in range(10)]
Just use list(range(10)) instead.
Where list comps get complicated is when you combine them. Nested list 
comps are not too bad, although they can get messy:
[len(s) for s in [str(x) for x in [2**n for n in range(10)]]]
That's the same as:
powers_of_two = [2**n for n in range(10)]
strings = [str(x) for x in powers_of_two]
lengths = [len(s) for s in strings]
But what do you make of this?
[a*b for a in range(3) for b in range(4)]
This is like a nested for-loop:
results = []
for a in range(3):
 for b in range(4):
 results.append(a*b)
Hope this helps.
-- 
Steven