python-peps/pep-3101.txt

PEP: 3101
Title: Advanced String Formatting
Version: $Revision$
Last-Modified: $Date$
Author: Talin <talin at acm.org>
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 16-Apr-2006
Python-Version: 3.0
Post-History: 28-Apr-2006, 6-May-2006, 10-Jun-2006


Abstract

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


Rationale

    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.


Specification

    The specification will consist of the following 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.

    - Specification of how formatting errors are handled.
    
    Note on string encodings: Since this PEP is being targeted
    at Python 3.0, it is assumed that all strings are unicode strings,
    and that the use of the word 'string' in the context of this
    document will generally refer to a Python 3.0 string, which is
    the same as Python 2.x unicode object.
    
    If it should happen that this functionality is backported to
    the 2.x series, then it will be necessary to handle both regular
    string as well as unicode objects.  All of the function call
    interfaces described in this PEP can be used for both strings
    and unicode objects, and in all cases there is sufficient
    information to be able to properly deduce the output string
    type (in other words, there is no need for two separate APIs).
    In all cases, the type of the template string dominates - that
    is, the result of the conversion will always result in an object
    that contains the same representation of characters as the
    input template string.


String Methods

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

        "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.


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 by doubling:

        "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 and Compound Field Names

    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.
    
    A compound field name is a combination of multiple simple field
    names in an expression:

        "My name is {0.name}".format(file('out.txt'))
        
    This example shows the use of the 'getattr' or 'dot' operator
    in a field expression. The dot operator allows an attribute of
    an input value to be specified as the field value.

    The types of expressions that can be used in a compound name
    have been deliberately limited in order to prevent potential
    security exploits resulting from the ability to place arbitrary
    Python expressions inside of strings. Only two operators are
    supported, the '.' (getattr) operator, and the '[]' (getitem)
    operator.
    
    An example of the 'getitem' syntax:
    
        "My name is {0[name]}".format(dict(name='Fred'))
    
    It should be noted that the use of 'getitem' within a string is
    much more limited than its normal use. In the above example, the
    string 'name' really is the literal string 'name', not a variable
    named 'name'. The rules for parsing an item key are the same as
    for parsing a simple name - in other words, if it looks like a
    number, then its treated as a number, if it looks like an
    identifier, then it is used as a string.
    
    It is not possible to specify arbitrary dictionary keys from
    within a format string.


Conversion Specifiers

    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 there is a
    standard set of conversion specifiers used for any object that
    does not override them.

    Conversion specifiers can themselves contain replacement fields.
    For example, a field whose field width is itself a parameter
    could be specified via:
    
        "{0:{1}}".format(a, b, c)
        
    Note that the doubled '}' at the end, which would normally be
    escaped, is not escaped in this case.  The reason is because
    the '{{' and '}}' syntax for escapes is only applied when used
    *outside* of a format field. Within a format field, the brace
    characters always have their normal meaning.

    The syntax for conversion specifiers is open-ended, since a class
    can override the standard conversion specifiers. In such cases,
    the format() method merely passes all of the characters between
    the first colon and the matching brace to the relevant underlying
    formatting method.


Standard Conversion Specifiers

    If an object does not define its own conversion specifiers, a
    standard set of conversion specifiers are used.  These are similar
    in concept to the conversion specifiers used by the existing '%'
    operator, however there are also a number of significant
    differences.  The standard conversion specifiers fall into three
    major categories: string conversions, integer conversions and
    floating point conversions.
    
    The general form of a standard conversion specifier is:

        [[fill]align][sign][width][.precision][type]

    The brackets ([]) indicate an optional element.
    
    Then the optional align flag can be one of the following:

        '<' - Forces the field to be left-aligned within the available
              space (This is the default.)
        '>' - Forces the field to be right-aligned within the
              available space.
        '=' - Forces the padding to be placed after the sign (if any)
              but before the digits. This is used for printing fields
              in the form '+000000120'.
              
    Note that unless a minimum field width is defined, the field
    width will always be the same size as the data to fill it, so
    that the alignment option has no meaning in this case.
              
    The optional 'fill' character defines the character to be used to
    pad the field to the minimum width.  The alignment flag must be
    supplied if the character is a number other than 0 (otherwise the
    character would be interpreted as part of the field width
    specifier). A zero fill character without an alignment flag
    implies an alignment type of '='.
    
    The 'sign' element can be one of the following:

        '+'  - indicates that a sign should be used for both
               positive as well as negative numbers
        '-'  - indicates that a sign should be used only for negative
               numbers (this is the default behaviour)
        ' '  - indicates that a leading space should be used on
               positive numbers
        '()' - indicates that negative numbers should be surrounded
               by parentheses

    'width' is a decimal integer defining the minimum field width. If
    not specified, then the field width will be determined by the
    content.

    The 'precision' is a decimal number indicating how many digits
    should be displayed after the decimal point in a floating point
    conversion. In a string conversion the field indicates how many
    characters will be used from the field content. The precision is
    ignored for integer conversions.
    
    Finally, the 'type' determines how the data should be presented.
    If the type field is absent, an appropriate type will be assigned
    based on the value to be formatted ('d' for integers and longs,
    'g' for floats, and 's' for everything else.)

    The available string conversion types are:

        's' - String format. Invokes str() on the object.
              This is the default conversion specifier type.
        'r' - Repr format. Invokes repr() on the object.

    There are several integer conversion types. All invoke int() on
    the object before attempting to format it.

    The available integer conversion types are:

        'b' - Binary. Outputs the number in base 2.
        'c' - Character. Converts the integer to the corresponding
              unicode character before printing.
        'd' - Decimal Integer. Outputs the number in base 10.
        'o' - Octal format. Outputs the number in base 8.
        'x' - Hex format. Outputs the number in base 16, using lower-
              case letters for the digits above 9.
        'X' - Hex format. Outputs the number in base 16, using upper-
              case letters for the digits above 9.

    There are several floating point conversion types. All invoke
    float() on the object before attempting to format it.

    The available floating point conversion types are:

        'e' - Exponent notation. Prints the number in scientific
              notation using the letter 'e' to indicate the exponent.
        'E' - Exponent notation. Same as 'e' except it uses an upper
              case 'E' as the separator character.
        'f' - Fixed point. Displays the number as a fixed-point
              number.
        'F' - Fixed point. Same as 'f'.
        'g' - General format. This prints the number as a fixed-point
              number, unless the number is too large, in which case
              it switches to 'e' exponent notation.
        'G' - General format. Same as 'g' except switches to 'E'
              if the number gets to large.
        'n' - Number. This is the same as 'g', except that it uses the
              current locale setting to insert the appropriate
              number separator characters.
        '%' - Percentage. Multiplies the number by 100 and displays
              in fixed ('f') format, followed by a percent sign.

    Objects are able to define their own conversion specifiers to
    replace the standard ones.  An example is the 'datetime' class,
    whose conversion specifiers might look something like 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 type signature for the cFormat function is as follows:
    
        cformat(template, format_hook, args, kwargs)

    The parameters to the cformat function are:

        -- The format template string.
        -- 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
    arguments:

       format_hook(value, conversion)

    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 field_hook will be called once per field. The field_hook may
    take one of two actions:
    
        1) Return a string or unicode object that is the result
           of the formatting operation.

        2) Return None, 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.


Error handling

    The string formatting system has two error handling modes, which
    are controlled by the value of a class variable:
    
       string.strict_format_errors = True
       
    The 'strict_format_errors' flag defaults to False, or 'lenient'
    mode. Setting it to True enables 'strict' mode. The current mode
    determines how errors are handled, depending on the type of the
    error.
    
    The types of errors that can occur are:
    
    1) Reference to a missing or invalid argument from within a
    field specifier. In strict mode, this will raise an exception.
    In lenient mode, this will cause the value of the field to be
    replaced with the string '?name?', where 'name' will be the
    type of error (KeyError, IndexError, or AttributeError).
    
    So for example:
    
        >>> string.strict_format_errors = False
        >>> print 'Item 2 of argument 0 is: {0[2]}'.format( [0,1] )
        "Item 2 of argument 0 is: ?IndexError?"
    
    2) Unused argument. In strict mode, this will raise an exception.
    In lenient mode, this will be ignored.
    
    3) Exception raised by underlying formatter. These exceptions
    are always passed through, regardless of the current mode.


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
    already.

    - 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
      on.
      
    Some specific aspects of the syntax warrant additional comments:
    
    1) Backslash character for escapes.  The original version of
    this PEP used backslash rather than doubling to escape a bracket.
    This worked because backslashes in Python string literals that
    don't conform to a standard backslash sequence such as '\n'
    are left unmodified. However, this caused a certain amount
    of confusion, and led to potential situations of multiple
    recursive escapes, i.e. '\\\\{' to place a literal backslash
    in front of a bracket.
    
    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
    submitted.
    

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.


References

    [1] Python Library Reference - String formating operations
    http://docs.python.org/lib/typesseq-strings.html

    [2] Python Library References - Template strings
    http://docs.python.org/lib/node109.html

    [3] [Python-3000] String formating operations in python 3k
        http://mail.python.org/pipermail/python-3000/2006-April/000285.html

    [4] Composite Formatting - [.Net Framework Developer's Guide]
        http://msdn.microsoft.com/library/en-us/cpguide/html/cpconcompositeformatting.asp?frame=true


Copyright

    This document has been placed in the public domain.


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