Python String Literal

A string is a sequence of characters. In other words, a string is basically a bunch of words and/or symbols. Although byte literal is one type of string literal in Python, the literal value of a byte literal is always considered as byte sequences or byte arrays.

Definition of Python String Literal

A string literal is a sequence of characters from any source characters.

stringliteral::=[stringprefix](shortstring | longstring)
    stringprefix::="r" | "u" | "R" | "U" | "f" | "F" | "fr" | "Fr" | "fR" | "FR" | "rf" | "rF" | "Rf" | "RF"
    shortstring::="'" shortstringitem* "'" | '"' shortstringitem* '"'
    longstring::="'''" longstringitem* "'''" | '"""' longstringitem* '"""'
    shortstringitem::=shortstringchar | stringescapeseq
    longstringitem::=longstringchar | stringescapeseq
    shortstringchar::=<any source character except "\" or newline or the quote>
    longstringchar::=<any source character except "\">
    stringescapeseq::="\" <any source character>

Features of Python String

The Python string of string literal in Python have some typical features.

Python String Notation

A Python string is denoted by quotes. The ways to denote a Python string are. Single Quotes, '...'e.g. 'A single quoted string' Double Quotes, "..."e.g. 'A double quoted string' Triple Quotes, '''...''' or """..."""e.g. '''A triple quoted string''', """A triple quoted string"""

Python String Catalogue

Python strings can be divided into shortstring and longstring. The difference betweens shortstring and longstring are: shortstringEnclosed by single or double quotes. Does not allow reserved character backslash \, unescaped NEWLINE, and the corresponding quote ' or " longstringEnclosed by triple quotes. Does not allow reserved character backslash \ only. Unescaped newlines and quotes are allowed (and are retained), except that three unescaped quotes in a row terminate the literal.

Python String Prefix

Python String Prefix is used to specify the rules to interpret the following string literal. Whitespace is not allowed between the string prefix and the rest of the string literal. The meanings of supported string prefixs are: r, RA raw form string literal. The escape sequence function is turned off. Backslash \ is treated as an ordinary literal character. u, UAn unicode string literal. This prefix is only used to restore support for the unicode legacy literal. f, FA formatted string literal. A brace enclosed Python expression is enclosed in the string literal for formating. Using brace {or }. in formatted string literal outside f-string expression part should be replaced by double braces {{ or }}. Besides backslash \ cannot be used in f-string expression part. fr, Fr, fR, FR, rf, rF, Rf, RFSince f-string expression part is evaluated separately, raw formatted string literals are possible. Because backslashes \ in string literals with prefix r or R are treated as literal characters. And backslashes \ in string literals with \u and \U are always not treated as literal characters. Since the Python 2.x's raw unicode literals behave differently from that of the Python 3.x's, the ur syntax is not supported.

Python Source Characters

The Python source character set is defined by the encoding declaration. The Python source character set is UTF-8 if no encoding declaration is given in the source file.

Escape Sequences Characters

The backslash (\) character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character. The recognized escape sequences are: \<NEWLINE>Physically literal line break, logically BACKSLASH and NEWLINE are ignored. \\Backslash (\) \'Single quote (') \"Double quote (") \aASCII Bell (BEL) \bASCII Backspace (BS) \fASCII Formfeed (FF) \nASCII Linefeed (LF) \rASCII Carriage Return (CR) \tASCII Horizontal Tab (TAB) \vASCII Vertical Tab (VT) \oooCharacter with octal value ooo \xhhCharacter with hex value hh Escape sequences only recognized in string literals are: \N{name}Character named name in the Unicode database \uxxxxCharacter with 16-bit hex value xxxx \UxxxxxxxxCharacter with 32-bit hex value xxxxxxxx

Python Unicode Literial

An unicode string literal is only used to restore support for the unicode legacy literal. The prefix u or U is used to denote an unicode string literal. Python reads program text as Unicode code points; the encoding of a source file can be given by an encoding declaration and defaults to UTF-8.

Source and Reference

• https://docs.python.org/3/reference/introduction.html
• https://docs.python.org/3/reference/lexical_analysis.html#literals
• https://docs.python.org/3/reference/expressions.html
• https://docs.python.org/2.0/ref/atom-literals.html
• https://docs.python.org/3/library/stdtypes.html#binary-sequence-types-bytes-bytearray-memoryview