PEP-498 updates: specify str.interpolate() and beef up some of the discussion text. Still needs work on the XXX marked sections.

pep-0498.txt

@@ -41,6 +41,9 @@ values. Some examples are::
   >>> f'He said his name is {name!r}.'
   "He said his name is 'Fred'."
 
+This PEP proposes a new method on the str type: str.interpolate. This
+method will be used to implement f-strings.
+
 A similar feature was proposed in PEP 215 [#]_. PEP 215 proposed to
 support a subset of Python expressions, and did not support the
 type-specific string formatting (the __format__ method) which was
@@ -139,6 +142,11 @@ order for this to work::
   >>> outer(42)()
   'x=42'
 
+In addition, using locals() or globals() introduces an information
+leak. A called routine that has access to the callers locals() or
+globals() has access to far more information than needed to do the
+string interpolation.
+
 Guido stated [#]_ that any solution to better string interpolation
 would not use locals() or globals().
 
@@ -185,21 +193,34 @@ Expressions cannot contain ':' or '!' outside of strings or parens,
 brackets, or braces. The exception is that the '!=' operator is
 special cased.
 
+str.interpolate()
+-----------------
+
+str.interpolate will be a new method. It takes one argument: a mapping
+of field names to values. This method is the same as str.format_map()
+[#]_, with one difference: it does not interpret the field_name [#]_
+in any way. The field name is only used to look up the replacement
+value in the supplied mapping object. Like str.format() and
+str.format_map(), it does interpret and apply the optional conversion
+character and format specifier. Thus, a field name may not contain the
+characters ':' or '}', nor the strings '!s' and '!r'.
+
 Code equivalence
 ----------------
 
-The exact code that is executed when converting expressions to strings
-is unspecified by this PEP. However, it is specified that once the
-expression is evaluated, the result's __format__() method will be
-called with the given format specifier.
+An f-string is evaluated at run time as a call to str.interpolate().
 
 For example, this code::
 
   f'abc{expr1:spec1}{expr2!r:spec2}def{expr3:!s}ghi'
 
-May be evaluated as::
+Will be be evaluated as::
+
+  'abc{expr1:spec1}{expr2!r:spec2}def{expr3:!s}ghi'.interpolate({'expr1': expr1, 'expr2': expr2, 'expr3': expr3})
+
+Note that the string on which interpolate() is being called is
+identical to the value of the f-string.
 
-  ''.join(['abc', expr1.__format__('spec1'), repr(expr2).__format__('spec2'), 'def', str(expr3).__format__(''), 'ghi'])
 
 Expression evaluation
 ---------------------
@@ -258,10 +279,11 @@ yields the value::
 
   'ab10cstr< hi >de'
 
-While the exact code that is executed when evaluating this f-string is
-not specified, one possible strategy is to evaluate::
+While the exact method of this runtime concatenation is unspecified,
+the above code might evaluate to::
 
-  ''.join(['ab', x.__format__(''), 'c', 'str<', y.__format__('^4'), '>', 'de'])
+
+  ''.join(['ab', '{x}'.interpolate({'x': x}), 'c', 'str<', 'str<{y:^4}>'.interpolate({'y': y}), 'de'])
 
 Error handling
 --------------
@@ -310,24 +332,20 @@ or::
     File "<stdin>", line 2, in <module>
   ValueError: Sign not allowed in string format specifier
 
-Leading whitespace in expressions is skipped
---------------------------------------------
+Leading and trailing whitespace in expressions is skipped
+---------------------------------------------------------
 
-Because expressions may begin with a left brace ('{'), there is a
-problem when parsing such expressions. For example::
+For ease of readability, leading and trailing whitespace in
+expressions is ignored. However, this does not affect the string or
+keys passed to str.interpolate().
 
-  >>> f'{{k:v for k, v in [(1, 2), (3, 4)]}}'
-  '{k:v for k, v in [(1, 2), (3, 4)]}'
+  >>> x = 100
+  >>> f'x = { x }'
+  'x = 100'
 
-In this case, the doubled left braces and doubled right braces are
-interpreted as single braces, and the string becomes just a normal
-string literal. There is no expression evaluation being performed.
+This would be evaluated as::
 
-To allow for expressions to begin with a left brace, whitespace
-characters at the beginning of an expression are skipped::
-
-  >>> f'{ {k:v for k, v in [(1, 2), (3, 4)]}}'
-  '{1: 2, 3: 4}'
+  'x = { x }'.interpolate({' x ': 100})
 
 Discussion
 ==========
@@ -337,14 +355,15 @@ Most of the discussions on python-ideas [#]_ focused on a few issues:
  - Whether to allow full Python expressions.
  - How to designate f-strings, and how to specify the location of
    expressions in them.
- - How to concatenate adjacent strings and f-strings.
 
 XXX: more on the above issues.
 
 Similar support in other languages
 ----------------------------------
 
-XXX: Placeholder.
+Wikipedia has a good discussion of string interpolation in other
+programming languages [#]_. This feature is implemented in many
+languages, with a variety of syntaxes and restrictions.
 
 Differences between f-string and str.format expressions
 -------------------------------------------------------
@@ -377,6 +396,25 @@ use variables as index values::
 See [#]_ for a further discussion. It was this observation that led to
 full Python expressions being supported in f-strings.
 
+Triple-quoted f-strings
+-----------------------
+
+Triple quoted f-strings are allowed. These strings are parsed just as
+normal triple-quoted strings are. After parsing, the normal f-string
+logic is applied, and str.interpolate() is called.
+
+Raw f-strings
+-------------
+
+Raw and f-strings may be combined. For example they could be used to
+build up regular expressions::
+
+  >>> header = 'Subject'
+  >>> fr'{header}:\s+'
+  'Subject:\\s+'
+
+In addition, raw f-strings may be combined with triple-quoted strings.
+
 No binary f-strings
 -------------------
 
@@ -385,6 +423,8 @@ combine 'f' with 'b' string literals. The primary problem is that an
 object's __format__() method may return Unicode data that is not
 compatible with a bytes string.
 
+#XXX: maybe allow this, but encode the output as ascii?
+
 !s and !r are redundant
 -----------------------
 
@@ -421,6 +461,27 @@ Lambdas may be used inside of parens::
   >>> f'{(lambda x: x*2)(3)}'
   '6'
 
+Future extensions:
+==================
+
+XXX: By using another leading character (say, 'i'), we could extend
+this proposal to cover internationalization and localization. The idea
+is that the string would be passed to some lookup function before
+.interpolate() is called on it:
+
+   >>> name = 'Eric'
+   >>> fi'Name: {name}'
+
+Could be translated as::
+
+   gettext.gettext('Name: {name}').interpolate({'name': name})
+
+If gettext.gettext() returned '{name} es mi nombre', then the
+resulting string would be 'Eric es mi nombre'.
+
+Any such internationalization work will be specified in an additional
+PEP.
+
 References
 ==========
 
@@ -445,6 +506,15 @@ References
 .. [#] Avoid locals() and globals()
        (https://mail.python.org/pipermail/python-ideas/2015-July/034701.html)
 
+.. [#] str.format_map() documentation
+       (https://docs.python.org/3/library/stdtypes.html#str.format_map)
+
+.. [#] Format string syntax
+       (https://docs.python.org/3/library/string.html#format-string-syntax)
+
+.. [#] Wikipedia article on string interpolation
+       (https://en.wikipedia.org/wiki/String_interpolation)
+
 .. [#] Start of python-ideas discussion
        (https://mail.python.org/pipermail/python-ideas/2015-July/034657.html)