$ curl cheat.sh/
#  Given a list of lists `t`,
#  
#  ```python
#  flat_list = [item for sublist in t for item in sublist]
#  ```
#  
#  which means:

 flat_list = []
 for sublist in t:
     for item in sublist:
         flat_list.append(item)

#  is faster than the shortcuts posted so far. (`t` is the list to
#  flatten.)
#  
#  Here is the corresponding function:

 flatten = lambda t: [item for sublist in t for item in sublist]

#  As evidence, you can use the `timeit` module in the standard library:

 $ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' '[item for sublist in t for item in sublist]'
 10000 loops, best of 3: 143 usec per loop
 $ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'sum(t, [])'
 1000 loops, best of 3: 969 usec per loop
 $ python -mtimeit -s't=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'reduce(lambda x,y: x+y,t)'
 1000 loops, best of 3: 1.1 msec per loop

#  Explanation: the shortcuts based on `+` (including the implied use in
#  `sum`) are, of necessity, `O(T**2)` when there are T sublists -- as
#  the intermediate result list keeps getting longer, at each step a new
#  intermediate result list object gets allocated, and all the items in
#  the previous intermediate result must be copied over (as well as a few
#  new ones added at the end). So, for simplicity and without actual loss
#  of generality, say you have T sublists of k items each: the first k
#  items are copied back and forth T-1 times, the second k items T-2
#  times, and so on; total number of copies is k times the sum of x for x
#  from 1 to T excluded, i.e., `k * (T**2)/2`.
#  
#  The list comprehension just generates one list, once, and copies each
#  item over (from its original place of residence to the result list)
#  also exactly once.
#  
#  [Alex Martelli] [so/q/952914] [cc by-sa 3.0]

$
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