WIP: String literals

Co-authored-by: Blaise Pabon <blaise@gmail.com>

Author: encukou

Doc/reference/expressions.rst

@@ -133,13 +133,18 @@ Literals
 
 Python supports string and bytes literals and various numeric literals:
 
-.. productionlist:: python-grammar
-   literal: `stringliteral` | `bytesliteral` | `NUMBER`
+.. grammar-snippet::
+   :group: python-grammar
+
+   literal: `strings` | `NUMBER`
 
 Evaluation of a literal yields an object of the given type (string, bytes,
 integer, floating-point number, complex number) with the given value.  The value
 may be approximated in the case of floating-point and imaginary (complex)
-literals.  See section :ref:`literals` for details.
+literals.
+See section :ref:`literals` for details.
+Seee section :ref:`string-concatenation` for details on ``strings``.
+
 
 .. index::
    triple: immutable; data; type
@@ -152,6 +157,45 @@ occurrence) may obtain the same object or a different object with the same
 value.
 
 
+.. _string-concatenation:
+
+String literal concatenation
+............................
+
+Multiple adjacent string or bytes literals (delimited by whitespace), possibly
+using different quoting conventions, are allowed, and their meaning is the same
+as their concatenation.  Thus, ``"hello" 'world'`` is equivalent to
+``"helloworld"``.
+
+Formally:
+
+.. grammar-snippet::
+   :group: python-grammar
+
+   strings: ( `STRING` | `fstring` | `tstring`)+
+
+Note that this feature is defined at the syntactical level, so it only works
+with literals.
+To concatenate string expressions at run time, the '+' operator may be used::
+
+   greeting = "Hello"
+   space = " "
+   name = "Blaise"
+   print(greeting + space + name)   # not: print(greeting space name)
+
+Also note that literal concatenation can freely mix raw strings,
+triple-quoted strings, and formatted or template string literals.
+However, bytes literals may not be combined with string literals of any kind.
+
+This feature can be used to reduce the number of backslashes
+needed, to split long strings conveniently across long lines, or even to add
+comments to parts of strings, for example::
+
+   re.compile("[A-Za-z_]"       # letter or underscore
+              "[A-Za-z0-9_]*"   # letter, digit or underscore
+             )
+
+
 .. _parenthesized:
 
 Parenthesized forms