Equivalent syntax regarding List returns List objects with non-similar list elements.

[AkshayDeogaonkar]

BPO	30137
Nosy	@stevendaprano

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-04-22.18:16:43.046>
created_at = <Date 2017-04-22.14:01:58.944>
labels = ['type-bug', 'invalid']
title = 'Equivalent syntax regarding List returns List objects with non-similar list elements.'
updated_at = <Date 2017-04-22.18:16:43.044>
user = 'https://bugs.python.org/AkshayDeogaonkar'

bugs.python.org fields:

activity = <Date 2017-04-22.18:16:43.044>
actor = 'steven.daprano'
assignee = 'none'
closed = True
closed_date = <Date 2017-04-22.18:16:43.046>
closer = 'steven.daprano'
components = []
creation = <Date 2017-04-22.14:01:58.944>
creator = 'Akshay Deogaonkar'
dependencies = []
files = []
hgrepos = []
issue_num = 30137
keywords = []
message_count = 2.0
messages = ['292120', '292132']
nosy_count = 2.0
nosy_names = ['steven.daprano', 'Akshay Deogaonkar']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30137'
versions = ['Python 3.6']

[AkshayDeogaonkar]

lst = [0,1,2,3,4]
print(lst[0:3]) #returns [0,1,2]
print(lst[:3]) #returns [0,1,2]

#Above two syntax returns same lists.

print(lst[0:3:-1]) #returns []
print(lst[:3:-1]) #returns [4]

#Here is a bug; what expected was that the both syntax would have
returned the similar lists; however, they didn't!

[stevendaprano]

The behaviour is as documented and is not a bug. When you have a three-argument extended slice, the starting and stopping values depend on whether the stride (step) is positive or negative. Although that's buried in a footnote to the table.

https://docs.python.org/3/library/stdtypes.html#common-sequence-operations

The current behaviour is necessary so that the common case of both start and stop being blank is supported correctly for negative stride:

py> "abcde"[::-1]
'edcba'

So with a positive stride, your first example lst[0:3:-1] starts at index 0, ends at index 3, with step -1. That is an empty slice.

But your second example lst[:3:-1] starts at the end of the list, index len(lst), ends at 3, with step -1. That is equivalent to lst[5:3:-1] which is not the same as your first example.