[Python-Dev] PEP 3101 Update

Talin talin at acm.org
Sun May 7 03:54:31 CEST 2006

I've updated PEP 3101 based on the feedback collected so far.
PEP: 3101
Title: Advanced String Formatting
Version: $Revision: 45928 $
Last-Modified: $Date: 2006-05-06 18:49:43 -0700 (Sat, 06 May 2006) $
Author: Talin <talin at acm.org>
Status: Draft
Type: Standards
Content-Type: text/plain
Created: 16-Apr-2006
Python-Version: 3.0
Post-History: 28-Apr-2006, 6-May-2006


     This PEP proposes a new system for built-in string formatting
     operations, intended as a replacement for the existing '%' string
     formatting operator.


     Python currently provides two methods of string interpolation:

     - The '%' operator for strings. [1]

     - The string.Template module. [2]

     The scope of this PEP will be restricted to proposals for built-in
     string formatting operations (in other words, methods of the
     built-in string type).

     The '%' operator is primarily limited by the fact that it is a
     binary operator, and therefore can take at most two arguments.
     One of those arguments is already dedicated to the format string,
     leaving all other variables to be squeezed into the remaining
     argument.  The current practice is to use either a dictionary or a
     tuple as the second argument, but as many people have commented
     [3], this lacks flexibility.  The "all or nothing" approach
     (meaning that one must choose between only positional arguments,
     or only named arguments) is felt to be overly constraining.

     While there is some overlap between this proposal and
     string.Template, it is felt that each serves a distinct need,
     and that one does not obviate the other.  In any case,
     string.Template will not be discussed here.


     The specification will consist of 4 parts:

     - Specification of a new formatting method to be added to the
       built-in string class.

     - Specification of a new syntax for format strings.

     - Specification of a new set of class methods to control the
       formatting and conversion of objects.

     - Specification of an API for user-defined formatting classes.

String Methods

     The build-in string class will gain a new method, 'format',
     which takes takes an arbitrary number of positional and keyword

         "The story of {0}, {1}, and {c}".format(a, b, c=d)

     Within a format string, each positional argument is identified
     with a number, starting from zero, so in the above example, 'a' is
     argument 0 and 'b' is argument 1.  Each keyword argument is
     identified by its keyword name, so in the above example, 'c' is
     used to refer to the third argument.

     The result of the format call is an object of the same type
     (string or unicode) as the format string.

Format Strings

     Brace characters ('curly braces') are used to indicate a
     replacement field within the string:

         "My name is {0}".format('Fred')

     The result of this is the string:

         "My name is Fred"

     Braces can be escaped using a backslash:

         "My name is {0} :-\{\}".format('Fred')

     Which would produce:

         "My name is Fred :-{}"

     The element within the braces is called a 'field'.  Fields consist
     of a 'field name', which can either be simple or compound, and an
     optional 'conversion specifier'.

     Simple field names are either names or numbers. If numbers, they
     must be valid base-10 integers; if names, they must be valid
     Python identifiers.  A number is used to identify a positional
     argument, while a name is used to identify a keyword argument.

     Compound names are a sequence of simple names seperated by

         "My name is {0.name} :-\{\}".format(dict(name='Fred'))

     Compound names can be used to access specific dictionary entries,
     array elements, or object attributes.  In the above example, the
     '{0.name}' field refers to the dictionary entry 'name' within
     positional argument 0.

     Each field can also specify an optional set of 'conversion
     specifiers' which can be used to adjust the format of that field.
     Conversion specifiers follow the field name, with a colon (':')
     character separating the two:

         "My name is {0:8}".format('Fred')

     The meaning and syntax of the conversion specifiers depends on the
     type of object that is being formatted, however many of the
     built-in types will recognize a standard set of conversion

     The conversion specifier consists of a sequence of zero or more
     characters, each of which can consist of any printable character
     except for a non-escaped '}'.

     Conversion specifiers can themselves contain replacement fields;
     this will be described in a later section.  Except for this
     replacement, the format() method does not attempt to intepret the
     conversion specifiers in any way; it merely passes all of the
     characters between the first colon ':' and the matching right
     brace ('}') to the various underlying formatters (described

Standard Conversion Specifiers

     For most built-in types, the conversion specifiers will be the
     same or similar to the existing conversion specifiers used with
     the '%' operator.  Thus, instead of '%02.2x", you will say

     There are a few differences however:

     - The trailing letter is optional - you don't need to say '2.2d',
       you can instead just say '2.2'.  If the letter is omitted, a
       default will be assumed based on the type of the argument.
       The defaults will be as follows:

         string or unicode object: 's'
         integer: 'd'
         floating-point number: 'f'
         all other types: 's'

     - Variable field width specifiers use a nested version of the {}
       syntax, allowing the width specifier to be either a positional
       or keyword argument:

         "{0:{1}.{2}d}".format(a, b, c)

     - The support for length modifiers (which are ignored by Python
       anyway) is dropped.

     For non-built-in types, the conversion specifiers will be specific
     to that type.  An example is the 'datetime' class, whose
     conversion specifiers are identical to the arguments to the
     strftime() function:

         "Today is: {0:%a %b %d %H:%M:%S %Y}".format(datetime.now())

Controlling Formatting

     A class that wishes to implement a custom interpretation of its
     conversion specifiers can implement a __format__ method:

     class AST:
         def __format__(self, specifiers):

     The 'specifiers' argument will be either a string object or a
     unicode object, depending on the type of the original format
     string.  The __format__ method should test the type of the
     specifiers parameter to determine whether to return a string or
     unicode object.  It is the responsibility of the __format__ method
     to return an object of the proper type.

     string.format() will format each field using the following steps:

      1) See if the value to be formatted has a __format__ method.  If
         it does, then call it.

      2) Otherwise, check the internal formatter within string.format
         that contains knowledge of certain builtin types.

      3) Otherwise, call str() or unicode() as appropriate.

User-Defined Formatting Classes

     There will be times when customizing the formatting of fields
     on a per-type basis is not enough.  An example might be an
     accounting application, which displays negative numbers in
     parentheses rather than using a negative sign.

     The string formatting system facilitates this kind of application-
     specific formatting by allowing user code to directly invoke
     the code that interprets format strings and fields.  User-written
     code can intercept the normal formatting operations on a per-field
     basis, substituting their own formatting methods.

     For example, in the aforementioned accounting application, there
     could be an application-specific number formatter, which reuses
     the string.format templating code to do most of the work. The
     API for such an application-specific formatter is up to the
     application; here are several possible examples:

         cell_format( "The total is: {0}", total )

         TemplateString( "The total is: {0}" ).format( total )

     Creating an application-specific formatter is relatively straight-
     forward.  The string and unicode classes will have a class method
     called 'cformat' that does all the actual work of formatting; The
     built-in format() method is just a wrapper that calls cformat.

     The parameters to the cformat function are:

         -- The format string (or unicode; the same function handles
         -- A callable 'format hook', which is called once per field
         -- A tuple containing the positional arguments
         -- A dict containing the keyword arguments

     The cformat function will parse all of the fields in the format
     string, and return a new string (or unicode) with all of the
     fields replaced with their formatted values.

     The format hook is a callable object supplied by the user, which
     is invoked once per field, and which can override the normal
     formatting for that field.  For each field, the cformat function
     will attempt to call the field format hook with the following

        format_hook(value, conversion, buffer)

     The 'value' field corresponds to the value being formatted, which
     was retrieved from the arguments using the field name.

     The 'conversion' argument is the conversion spec part of the
     field, which will be either a string or unicode object, depending
     on the type of the original format string.

     The 'buffer' argument is a Python array object, either a byte
     array or unicode character array.  The buffer object will contain
     the partially constructed string; the field hook is free to modify
     the contents of this buffer if needed.

     The field_hook will be called once per field. The field_hook may
     take one of two actions:

         1) Return False, indicating that the field_hook will not
            process this field and the default formatting should be
            used.  This decision should be based on the type of the
            value object, and the contents of the conversion string.

         2) Append the formatted field to the buffer, and return True.

Alternate Syntax

     Naturally, one of the most contentious issues is the syntax of the
     format strings, and in particular the markup conventions used to
     indicate fields.

     Rather than attempting to exhaustively list all of the various
     proposals, I will cover the ones that are most widely used

     - Shell variable syntax: $name and $(name) (or in some variants,
       ${name}).  This is probably the oldest convention out there, and
       is used by Perl and many others.  When used without the braces,
       the length of the variable is determined by lexically scanning
       until an invalid character is found.

       This scheme is generally used in cases where interpolation is
       implicit - that is, in environments where any string can contain
       interpolation variables, and no special subsitution function
       need be invoked.  In such cases, it is important to prevent the
       interpolation behavior from occuring accidentally, so the '$'
       (which is otherwise a relatively uncommonly-used character) is
       used to signal when the behavior should occur.

       It is the author's opinion, however, that in cases where the
       formatting is explicitly invoked, that less care needs to be
       taken to prevent accidental interpolation, in which case a
       lighter and less unwieldy syntax can be used.

     - Printf and its cousins ('%'), including variations that add a
       field index, so that fields can be interpolated out of order.

     - Other bracket-only variations.  Various MUDs (Multi-User
       Dungeons) such as MUSH have used brackets (e.g. [name]) to do
       string interpolation.  The Microsoft .Net libraries uses braces
       ({}), and a syntax which is very similar to the one in this
       proposal, although the syntax for conversion specifiers is quite
       different. [4]

     - Backquoting.  This method has the benefit of minimal syntactical
       clutter, however it lacks many of the benefits of a function
       call syntax (such as complex expression arguments, custom
       formatters, etc.).

     - Other variations include Ruby's #{}, PHP's {$name}, and so

     Some specific aspects of the syntax warrant additional comments:

     1) The use of the backslash character for escapes.  A few people
     suggested doubling the brace characters to indicate a literal
     brace rather than using backslash as an escape character.  This is
     also the convention used in the .Net libraries.  Here's how the
     previously-given example would look with this convention:

         "My name is {0} :-{{}}".format('Fred')

     One problem with this syntax is that it conflicts with the use of
     nested braces to allow parameterization of the conversion

         "{0:{1}.{2}}".format(a, b, c)

     (There are alternative solutions, but they are too long to go
     into here.)

     2) The use of the colon character (':') as a separator for
     conversion specifiers.  This was chosen simply because that's
     what .Net uses.

Sample Implementation

     A rough prototype of the underlying 'cformat' function has been
     coded in Python, however it needs much refinement before being

Backwards Compatibility

     Backwards compatibility can be maintained by leaving the existing
     mechanisms in place.  The new system does not collide with any of
     the method names of the existing string formatting techniques, so
     both systems can co-exist until it comes time to deprecate the
     older system.


     [1] Python Library Reference - String formating operations

     [2] Python Library References - Template strings

     [3] [Python-3000] String formating operations in python 3k

     [4] Composite Formatting - [.Net Framework Developer's Guide]


     This document has been placed in the public domain.

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