Python loops with multiple lists?

Question

<edit> Thanks to everyone who has answered so far. The zip and os.path.join are really helpful. Any suggestions on ways to list the counter in front, without doing something like this:

zip(range(len(files)), files, directories)

</edit>

Hi,

I'm in the process of learning Python, but I come from a background where the following pseudocode is typical:

directories = ['directory_0', 'directory_1', 'directory_2']
files = ['file_a', 'file_b', 'file_c']

for(i = 0; i < directories.length; i++) {
    print (i + 1) + '. ' + directories[i] + '/' + files[i] + '\n'
}

# Output:
# 1. directory_0/file_a
# 2. directory_1/file_b
# 3. directory_2/file_c

In Python, the way I would write the above right now, would be like this:

directories = ['directory_0', 'directory_1', 'directory_2']
files = ['file_a', 'file_b', 'file_c']

for i in range(len(directories)):
    print '%s. %s/%s' % ((i + 1), directories[i], files[i]

# Output:
# 1. directory_0/file_a
# 2. directory_1/file_b
# 3. directory_2/file_c

While reading Dive into Python, Mark Pilgrim says that using for loops for counters is "Visual Basic-style thinking" (Simple Counters). He goes on to show how to use loops with dictionaries, but never really addresses a python solution in regards to how for loop counters are typically used in other languages.

I was hoping somebody could show me how to properly write the above scenario in Python. Is it possible to do it a different way?

If I took out the incrementing line count, is it possible to just match the two lists together using some kind of list comprehension?

For example, if all I wanted from the output was this (no counters, is that possible with list comprehension):

# Output:
# directory_0/file_a
# directory_1/file_b
# directory_2/file_c

Thanks in advance for any help.

Answer 1

Try this:

directories = ['directory_0', 'directory_1', 'directory_2']
files = ['file_a', 'file_b', 'file_c']

for file, dir in zip(files, directories):
    print dir + '/' + file

To explain, the zip() function takes lists as input and returns a list of "zipped" tuples. so zip([1,2,3,4,5],[a,b,c,d,e]) would return [(1,a),(2,b) and so on.

You can then assign the members of the tuples to variables with the python for <var> in <list> syntax.

There are a million different ways to do what you are asking, but the above uses some more "pythonic" constructs to make the code a lot more readable (IMHO, anyway).

Answer 2

import os.path
for dir, file in zip(directories, files):
    print(os.path.join(dir, file))                      # for directories, files

you can have it as a list comprehension as well, creating list of string with print going after that]

with counter:

for i, (dir, file) in enumerate(zip(directories, files)):
    print(i, os.path.join(dir, file))

Answer 3

Building on Ryan's answer, you can do:

for fileDir in [dir + '/' + file for dir in directories for file in files]:
    print(fileDir)

Answer 4

If you want to add a counter to any for loop in Python you can use the enumerate() function:

listA = ["A", "B", "C", "D", "E"]
listB = ["a", "b", "c", "d", "e"]
for i, (a, b) in enumerate(zip(listA, listB)):
    print "%d) %s, %s" % (i, a, b)

gives the output:

0) A, a
1) B, b
2) C, c
3) D, d
4) E, e