Скачать или смотреть fix list slicing and string slicing to match python when step is negative

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Okay, let's dive deep into list and string slicing in Python, focusing particularly on the nuances of negative steps. This tutorial will provide a comprehensive understanding with detailed explanations and code examples.

*Understanding Slicing Basics*

Slicing is a powerful technique in Python for extracting portions of sequences (lists, strings, tuples). The general syntax for slicing is:



**`start`**: The index where the slice *begins*. If omitted, it defaults to the beginning of the sequence (0 for lists/strings).

*`stop`**: The index where the slice *ends (up to, but not including this index). If omitted, it defaults to the end of the sequence.

**`step`**: The increment between each index selected for the slice. If omitted, it defaults to 1. A positive step moves forward through the sequence, and a negative step moves backward.

*Positive Steps: The Familiar Territory*

Before tackling negative steps, let's quickly review slicing with positive steps:



*Negative Steps: Stepping Backwards*

The real fun starts when we use a negative step. This tells Python to traverse the sequence *backwards*. The behavior can seem a bit counterintuitive at first, but with a clear understanding of how `start` and `stop` interact, it becomes manageable.

Here are the key principles to keep in mind when using a negative `step`:

1. **Direction Matters**: The direction of the slice is determined by the sign of the `step`. A negative step means you are moving from right to left.

2. *`start` Must Be to the Right of `stop`**: For a negative step to produce a meaningful result, the `start` index should generally be *greater than the `stop` index (in terms of position within the sequence). If `start` is less than `stop`, the result will typically be an empty sequence (unless the sequence has negative indices).

3. **Default `start` and `stop` with Negative Steps**: This is where things get interesting.
If `step` is negative and `start` is ...

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