How to take the first N items from a generator or list? [duplicate] How to take the first N items from a generator or list? [duplicate] python python

How to take the first N items from a generator or list? [duplicate]


Slicing a list

top5 = array[:5]
  • To slice a list, there's a simple syntax: array[start:stop:step]
  • You can omit any parameter. These are all valid: array[start:], array[:stop], array[::step]

Slicing a generator

import itertoolstop5 = itertools.islice(my_list, 5) # grab the first five elements
  • You can't slice a generator directly in Python. itertools.islice() will wrap an object in a new slicing generator using the syntax itertools.islice(generator, start, stop, step)

  • Remember, slicing a generator will exhaust it partially. If you want to keep the entire generator intact, perhaps turn it into a tuple or list first, like: result = tuple(generator)


import itertoolstop5 = itertools.islice(array, 5)


In my taste, it's also very concise to combine zip() with xrange(n) (or range(n) in Python3), which works nice on generators as well and seems to be more flexible for changes in general.

# Option #1: taking the first n elements as a list[x for _, x in zip(xrange(n), generator)]# Option #2, using 'next()' and taking care for 'StopIteration'[next(generator) for _ in xrange(n)]# Option #3: taking the first n elements as a new generator(x for _, x in zip(xrange(n), generator))# Option #4: yielding them by simply preparing a function# (but take care for 'StopIteration')def top_n(n, generator):    for _ in xrange(n): yield next(generator)


matomo