Replace values in list using Python [duplicate]

Build a new list with a list comprehension:

new_items = [x if x % 2 else None for x in items]

You can modify the original list in-place if you want, but it doesn't actually save time:

items = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for index, item in enumerate(items):
    if not (item % 2):
        items[index] = None

Here are (Python 3.6.3) timings demonstrating the non-timesave:

In [1]: %%timeit
   ...: items = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
   ...: for index, item in enumerate(items):
   ...:     if not (item % 2):
   ...:         items[index] = None
   ...:
1.06 µs ± 33.7 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [2]: %%timeit
   ...: items = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
   ...: new_items = [x if x % 2 else None for x in items]
   ...:
891 ns ± 13.6 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

And Python 2.7.6 timings:

In [1]: %%timeit
   ...: items = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
   ...: for index, item in enumerate(items):
   ...:     if not (item % 2):
   ...:         items[index] = None
   ...: 
1000000 loops, best of 3: 1.27 µs per loop
In [2]: %%timeit
   ...: items = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
   ...: new_items = [x if x % 2 else None for x in items]
   ...: 
1000000 loops, best of 3: 1.14 µs per loop

ls = [x if (condition) else None for x in ls]

Here's another way:

>>> L = range (11)
>>> map(lambda x: x if x%2 else None, L)
[None, 1, None, 3, None, 5, None, 7, None, 9, None]

Riffing on a side question asked by the OP in a comment, i.e.:

what if I had a generator that yields the values from range(11) instead of a list. Would it be possible to replace values in the generator?

Sure, it's trivially easy...:

def replaceiniter(it, predicate, replacement=None):
  for item in it:
    if predicate(item): yield replacement
    else: yield item

Just pass any iterable (including the result of calling a generator) as the first arg, the predicate to decide if a value must be replaced as the second arg, and let 'er rip.

For example:

>>> list(replaceiniter(xrange(11), lambda x: x%2))
[0, None, 2, None, 4, None, 6, None, 8, None, 10]

>>> L = range (11)
>>> [ x if x%2 == 1 else None for x in L ]
[None, 1, None, 3, None, 5, None, 7, None, 9, None]

Replace values in list using Python [duplicate]

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