string.py

"""
Strings (`str`) are immutable sequences of Unicode code points.

Because strings are immutable in Python, any operation that modifies a string
(such as concatenation or slicing) creates a new string object in memory. Repeatedly
concatenating strings in a loop using `+` can lead to O(N^2) performance issues;
using `"".join()` on an iterable of strings is the standard and more efficient
O(N) alternative.
"""

# Module-level constants
_DELIMITER = " | "


def main() -> None:
    # Strings are some of the most robust data structures around
    content = "Ultimate Python study guide"

    # We can compute the length of a string just like all other data structures
    assert len(content) > 0

    # We can use range slices to get substrings from a string
    assert content[:8] == "Ultimate"
    assert content[9:15] == "Python"
    assert content[::-1] == "ediug yduts nohtyP etamitlU"

    # Like tuples, we cannot change the data in a string. However, we can
    # create a new string from existing strings
    new_content = f"{content.upper()}{_DELIMITER}{content.lower()}"
    assert _DELIMITER in new_content

    # We can split one string into a list of strings
    split_content = new_content.split(_DELIMITER)
    assert isinstance(split_content, list)
    assert len(split_content) == 2
    assert all(isinstance(item, str) for item in split_content)

    # A two-element list can be decomposed as two variables
    upper_content, lower_content = split_content
    assert upper_content.isupper() and lower_content.islower()

    # Notice that the data in `upper_content` and `lower_content` exists
    # in the `new_content` variable as expected
    assert upper_content in new_content
    assert new_content.startswith(upper_content)
    assert lower_content in new_content
    assert new_content.endswith(lower_content)

    # Notice that `upper_content` and `lower_content` are smaller in length
    # than `new_content` and have the same length as the original `content`
    # they were derived from
    assert len(upper_content) < len(new_content)
    assert len(lower_content) < len(new_content)
    assert len(upper_content) == len(lower_content) == len(content)

    # We can also join `upper_content` and `lower_content` back into one
    # string with the same contents as `new_content`. The `join` method is
    # useful for joining an arbitrary amount of text items together
    joined_content = _DELIMITER.join(split_content)
    assert isinstance(joined_content, str)
    assert new_content == joined_content


if __name__ == "__main__":
    main()