Use zip() and sum(): In [1]: l = [(1, 2), (3, 4), (5, 6), (7, 8), (9, 0)] In [2]: [sum(x) for x in zip(*l)] Out[2]: [25, 20] or: In [4]: map(sum, zip(*l)) Out[4]: [25, 20] timeit results: In [16]: l = [(1, 2), (3, 4), (5, 6), (7, 8), (9, 0)]*1000 In [17]: %timeit … Read more
it’s typically referred to as flattening a nested structure. >>> tupleOfTuples = ((1, 2), (3, 4), (5,)) >>> [element for tupl in tupleOfTuples for element in tupl] [1, 2, 3, 4, 5] Just to demonstrate efficiency: >>> import timeit >>> it = lambda: list(chain(*tupleOfTuples)) >>> timeit.timeit(it) 2.1475738355700913 >>> lc = lambda: [element for tupl in … Read more
Both work differently. The list comprehension version takes advantage of the special bytecode LIST_APPEND which calls PyList_Append directly for us. Hence it avoids an attribute lookup to list.append and a function call at the Python level. >>> def func_lc(): [x**2 for x in y] … >>> dis.dis(func_lc) 2 0 LOAD_CONST 1 (<code object <listcomp> at … Read more
Lets break it down. A simple list-comprehension: [x for x in collection] This is easy to understand if we break it into parts: [A for B in C] A is the item that will be in the resulting list B is each item in the collection C C is the collection itself. In this way, … Read more
It’s producing an identical list as b contains no elements at run-time. What you’d want it this: >>> a = [1, 2, 3, 3, 5, 9, 6, 2, 8, 5, 2, 3, 5, 7, 3, 5, 8] >>> b = [] >>> [b.append(item) for item in a if item not in b] [None, None, None, … Read more
_ is a standard placeholder name for ignored members in a for-loop and tuple assignment, e.g. [” for _ in myList] [a+d for a, _, _, d, _ in fiveTuples] BTW your list could be written without list comprehension (assuming you want to make a list of immutable members like strings, integers etc.). [”] * … Read more
General approaches There are three ordinary ways to approach the problem: by explicitly using a loop (normally a for loop, but while loops are also possible); by using a list comprehension (or dict comprehension, set comprehension, or generator expression as appropriate to the specific need in context); or by using the built-in map (results of … Read more
Python’s creator, Guido van Rossum, mentions this when he wrote about generator expressions that were uniformly built into Python 3: (emphasis mine) We also made another change in Python 3, to improve equivalence between list comprehensions and generator expressions. In Python 2, the list comprehension “leaks” the loop control variable into the surrounding scope: x … Read more
You have the order of your loops mixed up; they are considered nested from left to right, so for r in a[g] is the outer loop and executed first. Swap out the loops: print [r[‘n’] for g in good for r in a[g]] Now g is defined for the next loop, for r in a[g], … Read more