Understanding List Comprehension Vs Traditional Loop And Build

I'm trying to nail my understanding and best use of Lists, I have come across list comprehension and read plenty about them, but am choking on one particular burning question. Give

Solution 1:

List comprehension is simply a way of creating a list based on another list. (Or other iterable item)

For instance, if we have a list a = [1, 2, 5, 7], then we could create a list, b, containing the values of a doubled in two ways.

Without list comprehensions

b = []
for e in a:
    b.append(2*e)

With list comprehensions

b = [2*e for e in a]

There's nothing more to it than that. It's simply a nice syntax for building lists based on lists.

See also:

• Python: List Comprehensions
• List Comprehensions on (An Unofficial) Python Tutorial Wiki

Solution 2:

The first line is a list comprehension. The rest is not.

return [[row_times_column(m1, i, m2, j) for j in range(len(res[i]))]
     for i in range(len(res))]

Solution 3:

The following also uses list comprehension for matrix multiplication

defmatrix_mult(m1, m2):
  return [[sum(m1[i][k]*m2[k][j] for k inrange(len(m2))) for j inrange(len(m2[0]))] for i inrange(len(m1))]

Solution 4:

The rationale between list comprehension in Python is explained in PEP 202

List comprehensions provide a more concise way to create lists in situations where map() and filter() and/or nested loops would currently be used.

So there is really no "special trickery" behind the idea. It's just a syntactical extension, to put it as the PEP does.

HTH

Solution 5:

Mark Pilgrim does a much better job at explaining list comprehension than I could in his book Dive into Python. This book is really good for starting out in Python and just a good read in general. Recommended.

P.S.: There's list comprehension for Python 3 as well in Dive into Python 3