Problem in understanding Python list comprehensions

Maybe best explained with an example:

print "".join([e[1] * e[0] for e in elt])

is the short form of

x = []
for e in elt:
  x.append(e[1] * e[0])
print "".join(x)

List comprehensions are simply syntactic sugar for for loops, which make an expression out of a sequence of statements.

elt can be an arbitrary object, since you load it from pickles, and e likewise. The usage suggests that is it a sequence type, but it could just be anything that implements the sequence protocol.

The question itself has already been fully answered but I'd like to add that a list comprehension also supports filtering. Your original line

print "".join([e[1] * e[0] for e in elt])

could, as an example, become

print "".join([e[1] * e[0] for e in elt if len(e) == 2])

to only operate on items in elt that have two elements.

Firstly, you need to put http:// in front of the URL, ie:

handle = urllib.urlopen("http://www.pythonchallenge.com/pc/def/banner.p")

An expression [e for e in a_list] is a list comprehension which generates a list of values.

With Python strings, the * operator is used to repeat a string. Try typing in the commands one by one into an interpreter then look at data:

>>> data[0]
[(' ', 95)]

This shows us each line of data is a tuple containing two elements.

Thus the expression e[1] * e[0] is effectively the string in e[0] repeated e[1] times.

Hence the program draws a banner.

[e[1] * e[0] for e in elt] is a list comprehension, which evaluates to a list itself by looping through another list, in this case elt. Each element in the new list is e[1]*e[0], where e is the corresponding element in elt.