Revisions to How slicing in Python works

Active reading [<https://www.youtube.com/watch?v=1Dax90QyXgI&t=17m54s> ].

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edited Sep 26, 2019 at 8:22

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If you feel negative indices in slicing is confusing, here's a very easy way to think about it: just replace the negative index with len - index. So for example, replace -3 with len(list) - 3.

The best way to illustrate what slicing does internally is just show it in code that implements this operation:

def slice(list, start = None, end = None, step = 1):
  # takeTake care of missing start/end parameters
  start = 0 if start is None else start
  end = len(list) if end is None else end

  # takeTake care of negative start/end parameters
  start = len(list) + start if start < 0 else start
  end = len(list) + end if end < 0 else end

  # nowNow just execute a for-loop with start, end and step
  return [list[i] for i in range(start, end, step)]

If you feel negative indices in slicing is confusing, here's very easy way to think about it: just replace negative index with len - index. So for example, replace -3 with len(list) - 3.

The best way to illustrate what slicing does internally is just show it in code that implements this operation:

def slice(list, start = None, end = None, step = 1):
  # take care of missing start/end parameters
  start = 0 if start is None else start
  end = len(list) if end is None else end

  # take care of negative start/end parameters
  start = len(list) + start if start < 0 else start
  end = len(list) + end if end < 0 else end

  # now just execute for-loop with start, end and step
  return [list[i] for i in range(start, end, step)]

If you feel negative indices in slicing is confusing, here's a very easy way to think about it: just replace the negative index with len - index. So for example, replace -3 with len(list) - 3.

The best way to illustrate what slicing does internally is just show it in code that implements this operation:

def slice(list, start = None, end = None, step = 1):
  # Take care of missing start/end parameters
  start = 0 if start is None else start
  end = len(list) if end is None else end

  # Take care of negative start/end parameters
  start = len(list) + start if start < 0 else start
  end = len(list) + end if end < 0 else end

  # Now just execute a for-loop with start, end and step
  return [list[i] for i in range(start, end, step)]

added 2 characters in body

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edited May 21, 2018 at 23:16

Shital Shah

reputation score 70312
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If you feel negative indices in slicing is bit "unnatural"confusing, here's very easy way to think about it: just replace negative index with len - indexlen - index. So for example, replace -3 with len(list) - 3. I think

The best way to illustrate what slicing does internally is just write someshow it in code that will give exactly the same behaviourimplements this operation:

def slice(list, start = None, end = None, step = 1):
  # take care of missing start/end parameters
  start = 0 if start is None else start
  end = len(list) if end is None else end

  # take care of negative start/end parameters
  start = len(list) + start if start < 0 else start
  end = len(list) + end if end < 0 else end

  # now just execute for-loop with start, end and step
  return [list[i] for i in range(start, end, step)]

If you feel negative indices in slicing is bit "unnatural", here's very easy way to think about it: just replace negative index with len - index. So for example, replace -3 with len(list) - 3. I think best way to illustrate what slicing does internally is just write some code that will give exactly the same behaviour:

def slice(list, start = None, end = None, step = 1):
  # take care of missing start/end parameters
  start = 0 if start is None else start
  end = len(list) if end is None else end

  # take care of negative start/end parameters
  start = len(list) + start if start < 0 else start
  end = len(list) + end if end < 0 else end

  # now just execute for-loop with start, end and step
  return [list[i] for i in range(start, end, step)]

If you feel negative indices in slicing is confusing, here's very easy way to think about it: just replace negative index with len - index. So for example, replace -3 with len(list) - 3.

The best way to illustrate what slicing does internally is just show it in code that implements this operation:

def slice(list, start = None, end = None, step = 1):
  # take care of missing start/end parameters
  start = 0 if start is None else start
  end = len(list) if end is None else end

  # take care of negative start/end parameters
  start = len(list) + start if start < 0 else start
  end = len(list) + end if end < 0 else end

  # now just execute for-loop with start, end and step
  return [list[i] for i in range(start, end, step)]

deleted 9 characters in body

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edited Jan 23, 2018 at 0:17

Shital Shah

reputation score 70312
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22 gold badges
259
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Many people tend toIf you feel uncomfortable with negative indices in slicing. The solution for that is bit "unnatural", here's very easy way to think about it: just replace negative index with len - index. So for example, replace -3 with len(list) - 3. I think best way to illustrate what slicing does internally is just write some code that will give exactly the same behaviour:

def slice(list, start = None, end = None, step = 1):
  # take care of missing start/end parameters
  start = 0 if start is None else start
  end = len(list) if end is None else end

  # take care of negative start/end parameters
  start = len(list) + start if start < 0 else start
  end = len(list) + end if end < 0 else end

  # now just execute for-loop with start, end and step
  return [list[i] for i in range(start, end, step)]

Many people tend to feel uncomfortable with negative indices in slicing. The solution for that is very easy: just replace negative index with len - index. So for example, replace -3 with len(list) - 3. I think best way to illustrate what slicing does internally is just write some code that will give exactly the same behaviour:

def slice(list, start = None, end = None, step = 1):
  # take care of missing start/end parameters
  start = 0 if start is None else start
  end = len(list) if end is None else end

  # take care of negative start/end parameters
  start = len(list) + start if start < 0 else start
  end = len(list) + end if end < 0 else end

  # now just execute for-loop with start, end and step
  return [list[i] for i in range(start, end, step)]

If you feel negative indices in slicing is bit "unnatural", here's very easy way to think about it: just replace negative index with len - index. So for example, replace -3 with len(list) - 3. I think best way to illustrate what slicing does internally is just write some code that will give exactly the same behaviour:

def slice(list, start = None, end = None, step = 1):
  # take care of missing start/end parameters
  start = 0 if start is None else start
  end = len(list) if end is None else end

  # take care of negative start/end parameters
  start = len(list) + start if start < 0 else start
  end = len(list) + end if end < 0 else end

  # now just execute for-loop with start, end and step
  return [list[i] for i in range(start, end, step)]

added 15 characters in body

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edited Jan 8, 2018 at 20:45

Shital Shah

reputation score 70312
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259
259 silver badges
204
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deleted 10 characters in body

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edited Dec 19, 2017 at 6:20

Shital Shah

reputation score 70312
22
22 gold badges
259
259 silver badges
204
204 bronze badges

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answered Dec 19, 2017 at 6:12

Shital Shah

reputation score 70312
22
22 gold badges
259
259 silver badges
204
204 bronze badges

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