PEP: 3101 Title: Advanced String Formatting Version: $Revision$ Last-Modified: $Date$ Author: Talin Status: Draft Type: Standards 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. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: