Question

In Python, I really enjoy how concise an implementation can be when using list comprehension. I love to do concise list comprehensions this:

myList = [1, 5, 11, 20, 30, 35] #input data
bigNumbers = [x for x in myList if x > 10]

However, I often encounter more verbose implementations like this:

myList = [1, 5, 11, 20, 30, 35] #input data
bigNumbers = []
for i in xrange(0, len(myList)):
    if myList[i] > 10:
        bigNumbers.append(myList[i])

When a for loop only looks through one data structure (e.g. myList[]), there is usually a straightforward list comprehension statement that is equivalent to the loop.
With this in mind, is there a refactoring tool that converts verbose Python loops into concise list comprehension statements?

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Previous StackOverflow questions have asked for advice on transforming loops into list comprehension. But, I have yet to find a question about automatically converting loops into list comprehension expressions.

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Motivation: There are numerous ways to answer the question “what does it mean for code to be clean?” Personally, I find that making code concise and getting rid of some of the fluff tends to make code cleaner and more readable. Naturally there’s a line in the sand between “concise code” and “incomprehensible one-liners.” Still, I often find it satisfying to write and work with concise code.

Answer 1

2to3 is a refactoring tool that can perform arbitrary refactorings, as long as you can specify them with a syntactical pattern. The pattern you might want to look for is this

VARIABLE1 = []
for VARIABLE2 in EXPRESSION1:
    if EXPRESSION2:
        VARIABLE1.append(EXPRESSION3)

This can be refactored safely to

VARIABLE1 = [EXPRESSION3 for VARIABLE2 in EXPRESSION1 if EXPRESSION2]

In your specific example, this would give

bigNumbers = [myList[i] for i in xrange(0, len(myList)) if myList[i] > 10]

Then, you can have another refactoring that replaces xrange(0, N) with xrange(N),
and another one that replaces

[VARIABLE1[VARIABLE2] for VARIABLE2 in xrange(len(VARIABLE1)) if EXPRESSION1]

with

[VARIABLE3 for VARIABLE3 in VARIABLE1 if EXPRESSION1PRIME]

There are several problems with this refactoring:

• EXPRESSION1PRIME must be EXPRESSION1 with all occurrences of
  VARIABLE1[VARIABLE2] replaced by VARIABLE3. This is possible with
  2to3, but requires explicit code to do the traversal and replacement.
• EXPRESSION1PRIME then must not contain no further occurrences of
  VARIABLE1. This can also be checked with explicit code.
• One needs to come up with a name for VARIABLE3. You have chosen x;
  there is no reasonable way to have this done automatically. You could
  chose to recycle VARIABLE1 (i.e. i) for that, but that may be confusing
  as it suggests that i is still an index. It might work to pick a synthetic
  name, such as VARIABLE1_VARIABLE2 (i.e. myList_i), and check whether
  that’s not used otherwise.
• One needs to be sure that VARIABLE1[VARIABLE2] yields the same as you get
  when using iter(VARIABLE1). It’s not possible to do this automatically.

If you want to learn how to write 2to3 fixers, take a look at Lennart Regebro’s book.