9

What does the last line mean in the following code?

import pickle, urllib                                                                                                                                                     

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

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

My attempt to the problem:

  • "".join... uses join -method to empty text
  • e[1] * e[0] multiplies two subsequent values in the sequence, e
  • I am not sure what is e
  • I am not sure, what it means, when you have something before for -loop, like: e[1] * e[0] for e in elt
dbr
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Léo Léopold Hertz 준영
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6 Answers6

21

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.

Torsten Marek
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7

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.

Boris Verkhovskiy
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Andy Dent
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  • But switch the elements: e[0] = 95 and e[1] = ' '. –  Feb 01 '09 at 18:36
  • thanks for the correction - I am so used to doing the "sensible" thing of multiplying a string by an integer that I read it as being that way around, especially as that's how the data is stored! – Andy Dent Feb 01 '09 at 18:44
4

[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.

Nikhil
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2

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.

markhellewell
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1

join() is a string method, that works on a separator in new string

>>> ':'.join(['ab', 'cd'])
>>> 'ab:cd'

and list comprehension is not necessary there, generator would suffice

SilentGhost
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1

Andy's is a great answer!

If you want to see every step of the loop (with line-breaks) try this out:

for elt in data:
    for e in elt:
        print "e[0] == %s, e[1] == %d, which gives us:  '%s'" % (e[0], e[1], ''.join(e[1] * e[0]))
mechanical_meat
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