# Introduction A `str` in Python is an [immutable sequence][text sequence] of [Unicode code points][unicode code points]. These could include letters, diacritical marks, positioning characters, numbers, currency symbols, emoji, punctuation, space and line break characters, and more. Being immutable, a `str` object's value in memory doesn't change; methods that appear to modify a string return a new copy or instance of that `str` object. A `str` literal can be declared via single `'` or double `"` quotes. The escape `\` character is available as needed. ```python >>> single_quoted = 'These allow "double quoting" without "escape" characters.' >>> double_quoted = "These allow embedded 'single quoting', so you don't have to use an 'escape' character." >>> escapes = 'If needed, a \'slash\' can be used as an escape character within a string when switching quote styles won\'t work.' ``` Multi-line strings are declared with `'''` or `"""`. ```python >>> triple_quoted = '''Three single quotes or "double quotes" in a row allow for multi-line string literals. Line break characters, tabs and other whitespace are fully supported. You\'ll most often encounter these as "doc strings" or "doc tests" written just below the first line of a function or class definition. They\'re often used with auto documentation ✍ tools. ''' ``` Strings can be concatenated using the `+` operator. This method should be used sparingly, as it is not very performant or easily maintained. ```python language = "Ukrainian" number = "nine" word = "дев'ять" sentence = word + " " + "means" + " " + number + " in " + language + "." >>> print(sentence) ... "дев'ять means nine in Ukrainian." ``` If a `list`, `tuple`, `set` or other collection of individual strings needs to be combined into a single `str`, [`.join()`][str-join], is a better option: ```python # str.join() makes a new string from the iterables elements. >>> chickens = ["hen", "egg", "rooster"] # Lists are iterable. >>> ' '.join(chickens) 'hen egg rooster' # Any string can be used as the joining element. >>> ' :: '.join(chickens) 'hen :: egg :: rooster' >>> ' 🌿 '.join(chickens) 'hen 🌿 egg 🌿 rooster' # Any iterable can be used as input. >>> flowers = ("rose", "daisy", "carnation") # Tuples are iterable. >>> '*-*'.join(flowers) 'rose*-*daisy*-*carnation' >>> flowers = {"rose", "daisy", "carnation"} # Sets are iterable, but output order is not guaranteed. >>> '*-*'.join(flowers) 'rose*-*carnation*-*daisy' >>> phrase = "This is my string" # Strings are iterable, but be careful! >>> '..'.join(phrase) 'T..h..i..s.. ..i..s.. ..m..y.. ..s..t..r..i..n..g' # Separators are inserted **between** elements, but can be any string (including spaces). # This can be exploited for interesting effects. >>> under_words = ['under', 'current', 'sea', 'pin', 'dog', 'lay'] >>> separator = ' ⤴️ under' >>> separator.join(under_words) 'under ⤴️ undercurrent ⤴️ undersea ⤴️ underpin ⤴️ underdog ⤴️ underlay' # The separator can be composed different ways, as long as the result is a string. >>> upper_words = ['upper', 'crust', 'case', 'classmen', 'most', 'cut'] >>> separator = ' 🌟 ' + upper_words[0] >>> separator.join(upper_words) 'upper 🌟 uppercrust 🌟 uppercase 🌟 upperclassmen 🌟 uppermost 🌟 uppercut' ``` Code points within a `str` can be referenced by `0-based index` number from the left: ```python creative = '창의적인' >>> creative[0] '창' >>> creative[2] '적' >>> creative[3] '인' ``` Indexing also works from the right, starting with a `-1-based index`: ```python creative = '창의적인' >>> creative[-4] '창' >>> creative[-2] '적' >>> creative[-1] '인' ``` There is no separate “character” or "rune" type in Python, so indexing a string produces a new `str` of length 1: ```python >>> website = "exercism" >>> type(website[0]) >>> len(website[0]) 1 >>> website[0] == website[0:1] == 'e' True ``` Substrings can be selected via _slice notation_, using [`[:stop:]`][common sequence operations] to produce a new string. Results exclude the `stop` index. If no `start` is given, the starting index will be 0. If no `stop` is given, the `stop` index will be the end of the string. ```python moon_and_stars = '🌟🌟🌙🌟🌟⭐' sun_and_moon = '🌞🌙🌞🌙🌞🌙🌞🌙🌞' >>> moon_and_stars[1:4] '🌟🌙🌟' >>> moon_and_stars[:3] '🌟🌟🌙' >>> moon_and_stars[3:] '🌟🌟⭐' >>> moon_and_stars[:-1] '🌟🌟🌙🌟🌟' >>> moon_and_stars[:-3] '🌟🌟🌙' >>> sun_and_moon[::2] '🌞🌞🌞🌞🌞' >>> sun_and_moon[:-2:2] '🌞🌞🌞🌞' >>> sun_and_moon[1:-1:2] '🌙🌙🌙🌙' ``` Strings can also be broken into smaller strings via [`.split()`][str-split], which will return a `list` of substrings. The list can then be further indexed or split, if needed. Using `.split()` without any arguments will split the string on whitespace. ```python >>> cat_ipsum = "Destroy house in 5 seconds mock the hooman." >>> cat_ipsum.split() ... ['Destroy', 'house', 'in', '5', 'seconds', 'mock', 'the', 'hooman.'] >>> cat_ipsum.split()[-1] 'hooman.' >>> cat_words = "feline, four-footed, ferocious, furry" >>> cat_words.split(', ') ... ['feline', 'four-footed', 'ferocious', 'furry'] ``` Separators for `.split()` can be more than one character. The **whole string** is used for split matching. ```python >>> colors = """red, orange, green, purple, yellow""" >>> colors.split(',\n') ['red', 'orange', 'green', 'purple', 'yellow'] ``` Strings support all [common sequence operations][common sequence operations]. Individual code points can be iterated through in a loop via `for item in `. Indexes _with_ items can be iterated through in a loop via `for index, item in enumerate()`. ```python >>> exercise = 'လေ့ကျင့်' # Note that there are more code points than perceived glyphs or characters >>> for code_point in exercise: ... print(code_point) ... လ ေ ့ က ျ င ် ့ # Using enumerate will give both the value and index position of each element. >>> for index, code_point in enumerate(exercise): ... print(index, ": ", code_point) ... 0 : လ 1 : ေ 2 : ့ 3 : က 4 : ျ 5 : င 6 : ် 7 : ့ ``` [common sequence operations]: https://docs.python.org/3/library/stdtypes.html#common-sequence-operations [str-join]: https://docs.python.org/3/library/stdtypes.html#str.join [str-split]: https://docs.python.org/3/library/stdtypes.html#str.split [text sequence]: https://docs.python.org/3/library/stdtypes.html#text-sequence-type-str [unicode code points]: https://stackoverflow.com/questions/27331819/whats-the-difference-between-a-character-a-code-point-a-glyph-and-a-grapheme