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The main content of this article is to explain "will Python slices index out of bounds?", interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Next, let the editor take you to learn "will Python slices index out of bounds?"

Slicing is mainly used in sequence objects to intercept the contents of an index according to the index interval.

The writing form of the slice: [I: iTunn: M]; where I is the starting index value of the slice, which can be omitted when it is the first place in the list; iRecn is the end position of the slice, which can be omitted when it is the last position of the list; m may not be provided, the default value is 1, and 0 is not allowed. When m is negative, the list is flipped.

The basic meaning of slicing is that it is caused by the first bit of the sequence, takes the last n-bit element to the right, and filters by m interval.

Here are some representative examples that basically cover the main points of slicing syntax:

# @ Python cat li = [1,4pyrrine 5pje 6je 7,9,11,14,16] # the following words can represent the entire list, where X > = len (li) li [0li X] = = li [0:] = = li [: X] = = li [:] = = li [:] = li [- X:] = li [- X:] li [1:5] = [4LING 5J 6J 7] # from 1 Take 5-1 bit element li [- 1: 5:2] = = [4jue 6] # from 1, take 5-1 bit element, filter Li [- 1:] = = [16] # take the penultimate element li [- 4 2] = = [9,11] # from the penultimate fourth Take-2-(- 4) = 2-bit element li [:-2] = = li [- len (li):-2] = = [1 len (li):-2] = # start from scratch, take-2-(- len (li)) = 7-bit element # when the step size is negative, the list is flipped first, and then intercept li [::-1] = [16pr 14pr 11pr 9pm 7pm 6pr 5J 4J 1] # flip the entire list li [::-2] = [16pence11pencr5je 5j1] # Flip the entire list. Then filter li [:-5 len (li)) = 4-bit element li [:-5 (- len (li) = 4-bit element 0li [:: 0] # the step size of slice cannot be 0li [:: 0] # error report (ValueError: slice step cannot be zero)

Although languages such as CAccord Clipper +, Java, and JavaScript also support some "slicing" features, such as intercepting fragments of arrays or strings, they do not have a universal support at the syntax level.

According to Wikipedia, Fortran is the first language to support slicing syntax (1966), while Python is one of the most representative languages.

Support for slicing in major programming languages

In addition, languages like Perl, Ruby, Go, and Rust, although they also have slicing, are not as flexible and free as Python (because it supports step, negative indexes, and default indexes).

The form of slicing syntax in programming languages

The basic usage of slicing can meet most of the needs, but Python slicing also has some advanced uses, such as: slicing placeholder usage (to achieve list assignment, deletion and stitching), custom object slicing function, iterator slicing (itertools.islice ()), file object slicing, and so on. Related reading: Python's comprehensive interpretation of advanced feature slices

So much for the introduction and review of slicing.

Let's move on to the question of the title of the article: why doesn't the slicing syntax of Python go out of bounds?

When we take a value based on a single index, if the index is out of bounds, we get an error: "IndexError: list index out of range".

> li = [1,2] > > li [5] Traceback (most recent call last): File ", line 1, in IndexError: list index out of range

For a non-empty sequence object, assuming its length is length, its valid index value is from 0 to (length-1). If the negative index is also taken into account, the valid interval of a single index value is the [- length, length-1] closed interval.

However, when the index in the Python slice is out of this range, the program does not report an error.

> li = [1,2] > > li [1:5] # right index exceeds [2] > li [5:6] # index exceeds []

In fact, this phenomenon is described in the official documentation:

The slice of s from i to j is defined as the sequence of items with index k such that i > > li = [1,2] > li [1:5] # is equivalent to li [1:2] [2] > li [5:6] # is equivalent to li [2:2] []

To sum up: the Python interpreter blocks operations that may cause the index to cross the bounds, you can write freely, but the end result will be limited to a legal index range.

For this phenomenon, I am actually a little confused, why does Python not directly report that the index is out of bounds, why modify the boundary value of the slice, and why must a value be returned, even if the value may be an empty sequence?

When we use "li [5:6]", at least literally, it means "take the index from 5 to 6", which is like "take out the sixth and seventh books from left to right on the shelf."

If the program follows our instructions truthfully, it should report an error and say, "Sorry, there are not enough books on the shelf."

To be honest, I didn't find any explanation in this respect, and this article is not intended to give you any unique insights into the design of popular science Python. On the contrary, one of the main purposes of this article is to get a reply.

In the Go language, when it encounters the same scenario, it reports an error of "runtime error: slice bounds out of range".

In the Rust language, when it encounters the same scenario, it reports an error of "byte index 5 is out of bounds of."

Among other languages that support slicing syntax, there may be the same design as Python. However, I don't know if there is any (shallow knowledge)....

Finally, go back to the question in the title, "Why do slices of Python not index out of bounds?" There are actually two questions I want to ask:

Why can Python return the result when the index in the slicing syntax is out of bounds, and what is the calculation principle of the return result?

Why does Python's slicing syntax allow indexes to go out of bounds, and why not design them to throw index errors?

The official document is clear about the answer to the first question.

At this point, I believe that everyone on the "Python slices will index out of bounds" have a deeper understanding, might as well to the actual operation of it! Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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