String literals are described by the following lexical definitions:
|stringliteral||::=||[stringprefix](shortstring | longstring)|
|stringprefix||::=||"r" | "u" | "ur" | "R" | "U" | "UR" | "Ur" | "uR"|
|shortstring||::=||"'" shortstringitem* "'" | '"' shortstringitem* '"'|
|longstring||::=||"'''" longstringitem* "'''"|
|shortstringitem||::=||shortstringchar | escapeseq|
|longstringitem||::=||longstringchar | escapeseq|
|shortstringchar||::=||<any ASCII character except "\" or newline or the quote>|
|longstringchar||::=||<any ASCII character except "\">|
|escapeseq||::=||"\" <any ASCII character>|
One syntactic restriction not indicated by these productions is that whitespace is not allowed between the stringprefix and the rest of the string literal.
In plain English: String literals can be enclosed in matching single
') or double quotes (
"). They can also be
enclosed in matching groups of three single or double quotes (these
are generally referred to as triple-quoted strings). The
\) character is used to escape characters that
otherwise have a special meaning, such as newline, backslash itself,
or the quote character. String literals may optionally be prefixed
with a letter "r" or "R"; such strings are called
raw strings and use different rules for interpreting
backslash escape sequences. A prefix of "u" or "U"
makes the string a Unicode string. Unicode strings use the Unicode character
set as defined by the Unicode Consortium and ISO 10646. Some additional
escape sequences, described below, are available in Unicode strings.
The two prefix characters may be combined; in this case, "u" must
appear before "r".
In triple-quoted strings,
unescaped newlines and quotes are allowed (and are retained), except
that three unescaped quotes in a row terminate the string. (A
``quote'' is the character used to open the string, i.e. either
Unless an "r" or "R" prefix is present, escape sequences in strings are interpreted according to rules similar to those used by Standard C. The recognized escape sequences are:
||Single quote (
||Double quote (
||ASCII Bell (BEL)|
||ASCII Backspace (BS)|
||ASCII Formfeed (FF)|
||ASCII Linefeed (LF)|
||Character named name in the Unicode database (Unicode only)|
||ASCII Carriage Return (CR)|
||ASCII Horizontal Tab (TAB)|
||Character with 16-bit hex value xxxx (Unicode only)||(1)|
||Character with 32-bit hex value xxxxxxxx (Unicode only)||(2)|
||ASCII Vertical Tab (VT)|
||ASCII character with octal value ooo||(3)|
||ASCII character with hex value hh||(4)|
Unlike Standard C, all unrecognized escape sequences are left in the string unchanged, i.e., the backslash is left in the string. (This behavior is useful when debugging: if an escape sequence is mistyped, the resulting output is more easily recognized as broken.) It is also important to note that the escape sequences marked as ``(Unicode only)'' in the table above fall into the category of unrecognized escapes for non-Unicode string literals.
When an "r" or "R" prefix is present, a character
following a backslash is included in the string without change, and all
backslashes are left in the string. For example, the string literal
r"\n" consists of two characters: a backslash and a lowercase
"n". String quotes can be escaped with a backslash, but the
backslash remains in the string; for example,
r"\"" is a valid string
literal consisting of two characters: a backslash and a double quote;
r"\" is not a valid string literal (even a raw string cannot
end in an odd number of backslashes). Specifically, a raw
string cannot end in a single backslash (since the backslash would
escape the following quote character). Note also that a single
backslash followed by a newline is interpreted as those two characters
as part of the string, not as a line continuation.
When an "r" or "R" prefix is used in conjunction
with a "u" or "U" prefix, then the
escape sequence is processed while all other backslashes are
left in the string. For example, the string literal
ur"\u0062\n" consists of three Unicode characters: `LATIN
SMALL LETTER B', `REVERSE SOLIDUS', and `LATIN SMALL LETTER N'.
Backslashes can be escaped with a preceding backslash; however, both
remain in the string. As a result,
\uXXXX escape sequences
are only recognized when there are an odd number of backslashes.
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