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Updated PEP 3101 to incorporate latest feedback, and simplify even further. Also added additional explanation of custom formatting classes.

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Talin 2007-07-24 23:36:34 +00:00
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pep-3101.txt
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@ -141,7 +141,7 @@ Format Strings
Simple and Compound Field Names
Simple field names are either names or numbers. If numbers, they
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.
@ -152,44 +152,37 @@ Simple and Compound Field Names
"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
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.
Another limitation that is defined to limit potential security
issues is that field names or attribute names beginning with an
underscore are disallowed. This enforces the common convention
that names beginning with an underscore are 'private'.
Unlike some other programming languages, you cannot embed arbitrary
expressions in format strings. This is by design - the types of
expressions that you can use is deliberately limited. Only two operators
are supported: the '.' (getattr) operator, and the '[]' (getitem)
operator. The reason for allowing these operators is that they dont'
normally have side effects in non-pathological code.
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 very simple.
It should be noted that the use of 'getitem' within a format string
is much more limited than its conventional usage. 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 very simple.
If it starts with a digit, then its treated as a number, otherwise
it is used as a string.
It is not possible to specify arbitrary dictionary keys from
within a format string.
Implementation note: The implementation of this proposal is
Implementation note: The implementation of this proposal is
not required to enforce the rule about a name being a valid
Python identifier. Instead, it will rely on the getattr function
of the underlying object to throw an exception if the identifier
is not legal. The format function will have a minimalist parser
which only attempts to figure out when it is "done" with an
identifier (by finding a '.' or a ']', or '}', etc.) The only
exception to this laissez-faire approach is that, by default,
strings are not allowed to have leading underscores.
identifier (by finding a '.' or a ']', or '}', etc.).
Conversion Specifiers
@ -215,11 +208,11 @@ Conversion Specifiers
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
*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,
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.
@ -248,7 +241,7 @@ Standard Conversion Specifiers
'>' - 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
but before the digits. This is used for printing fields
in the form '+000000120'.
'^' - Forces the field to be centered within the available
space.
@ -261,7 +254,7 @@ Standard Conversion Specifiers
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
specifier). A zero fill character without an alignment flag
implies an alignment type of '='.
The 'sign' element can be one of the following:
@ -269,20 +262,20 @@ Standard Conversion Specifiers
'+' - 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)
numbers (this is the default behavior)
' ' - 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
'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
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.
@ -292,11 +285,11 @@ Standard Conversion Specifiers
The available string conversion types are:
's' - String format. Invokes str() on the object.
's' - String format. Invokes str() on the object.
This is the default conversion specifier type.
'r' - Repr format. Invokes repr() on the object.
'r' - Repr format. Invokes repr() on the object.
There are several integer conversion types. All invoke int() on
There are several integer conversion types. All invoke int() on
the object before attempting to format it.
The available integer conversion types are:
@ -311,7 +304,7 @@ Standard Conversion Specifiers
'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
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:
@ -380,97 +373,125 @@ User-Defined Formatting
format engine can be obtained through the 'Formatter' class that
lives in the 'string' module. This class takes additional options
which are not accessible via the normal str.format method.
An application can create their own Formatter instance which has
customized behavior, either by setting the properties of the
Formatter instance, or by subclassing the Formatter class.
An application can subclass the Formatter class to create their
own customized formatting behavior.
The PEP does not attempt to exactly specify all methods and
properties defined by the Formatter class; Instead, those will be
defined and documented in the initial implementation. However, this
defined and documented in the initial implementation. However, this
PEP will specify the general requirements for the Formatter class,
which are listed below.
Formatter Creation and Initialization
The Formatter class takes a single initialization argument, 'flags':
Formatter(flags=0)
The 'flags' argument is used to control certain subtle behavioral
differences in formatting that would be cumbersome to change via
subclassing. The flags values are defined as static variables
in the "Formatter" class:
Formatter.ALLOW_LEADING_UNDERSCORES
By default, leading underscores are not allowed in identifier
lookups (getattr or getitem). Setting this flag will allow
this.
Formatter.CHECK_UNUSED_POSITIONAL
If this flag is set, the any positional arguments which are
supplied to the 'format' method but which are not used by
the format string will cause an error.
Formatter.CHECK_UNUSED_NAME
If this flag is set, the any named arguments which are
supplied to the 'format' method but which are not used by
the format string will cause an error.
Although string.format() does not directly use the Formatter class
to do formatting, both use the same underlying implementation. The
reason that string.format() does not use the Formatter class directly
is because "string" is a built-in type, which means that all of its
methods must be implemented in C, whereas Formatter is a Python
class. Formatter provides an extensible wrapper around the same
C functions as are used by string.format().
Formatter Methods
The methods of class Formatter are as follows:
The Formatter class takes no initialization arguments:
fmt = Formatter()
The public API methods of class Formatter are as follows:
-- format(format_string, *args, **kwargs)
-- vformat(format_string, args, kwargs)
-- get_positional(args, index)
-- get_named(kwds, name)
-- format_field(value, conversion)
'format' is the primary API method. It takes a format template,
and an arbitrary set of positional and keyword argument. 'format'
'format' is the primary API method. It takes a format template,
and an arbitrary set of positional and keyword argument. 'format'
is just a wrapper that calls 'vformat'.
'vformat' is the function that does the actual work of formatting. It
'vformat' is the function that does the actual work of formatting. It
is exposed as a separate function for cases where you want to pass in
a predefined dictionary of arguments, rather than unpacking and
repacking the dictionary as individual arguments using the '*args' and
'**kwds' syntax. 'vformat' does the work of breaking up the format
template string into character data and replacement fields. It calls
the 'get_positional' and 'get_index' methods as appropriate.
'**kwds' syntax. 'vformat' does the work of breaking up the format
template string into character data and replacement fields. It calls
the 'get_positional' and 'get_index' methods as appropriate (described
below.)
Note that the checking of unused arguments, and the restriction on
leading underscores in attribute names are also done in this function.
Formatter defines the following overridable methods:
-- get_positional(args, index)
-- get_named(kwds, name)
-- check_unused_args(used_args, args, kwargs)
-- format_field(value, conversion)
'get_positional' and 'get_named' are used to retrieve a given field
value. For compound field names, these functions are only called for
value. For compound field names, these functions are only called for
the first component of the field name; Subsequent components are
handled through normal attribute and indexing operations. So for
example, the field expression '0.name' would cause 'get_positional' to
be called with the list of positional arguments and a numeric index of
0, and then the standard 'getattr' function would be called to get the
'name' attribute of the result.
handled through normal attribute and indexing operations.
So for example, the field expression '0.name' would cause
'get_positional' to be called with the parameter 'args' set to the
list of positional arguments to vformat, and 'index' set to zero;
the returned value would then be passed to the standard 'getattr'
function to get the 'name' attribute.
If the index or keyword refers to an item that does not exist, then an
IndexError/KeyError will be raised.
'check_unused_args' is used to implement checking for unused arguments
if desired. The arguments to this function is the set of all argument
keys that were actually referred to in the format string (integers for
positional arguments, and strings for named arguments), and a reference
to the args and kwargs that was passed to vformat. The intersection
of these two sets will be the set of unused args. 'check_unused_args'
is assumed to throw an exception if the check fails.
'format_field' actually generates the text for a replacement field.
The 'value' argument corresponds to the value being formatted, which
was retrieved from the arguments using the field name. The
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.
To get a better understanding of how these functions relate to each
other, here is pseudocode that explains the general operation of
vformat:
def vformat(format_string, args, kwargs):
# Output buffer and set of used args
buffer = StringIO.StringIO()
used_args = set()
# Tokens are either format fields or literal strings
for token in self.parse(format_string):
if is_format_field(token):
field_spec, conversion_spec = token.rsplit(":", 2)
# 'first_part' is the part before the first '.' or '['
first_part = get_first_part(token)
used_args.add(first_part)
if is_positional(first_part):
value = self.get_positional(args, first_part)
else:
value = self.get_named(kwargs, first_part)
# Handle [subfield] or .subfield
for comp in components(token):
value = resolve_subfield(value, comp)
Note: The final implementation of the Formatter class may define
additional overridable methods and hooks. In particular, it may be
that 'vformat' is itself a composition of several additional,
overridable methods. (Depending on whether it is convenient to the
implementor of Formatter.)
# Write out the converted value
buffer.write(format_field(value, conversion))
else:
buffer.write(token)
self.check_unused_args(used_args, args, kwargs)
return buffer.getvalue()
Note that the actual algorithm of the Formatter class may not be the
one presented here. In particular, the final implementation of
the Formatter class may define additional overridable methods and
hooks. Also, the final implementation will be written in C.
Customizing Formatters
@ -511,15 +532,15 @@ Customizing Formatters
It would also be possible to create a 'smart' namespace formatter
that could automatically access both locals and globals through
snooping of the calling stack. Due to the need for compatibility
snooping of the calling stack. Due to the need for compatibility
the different versions of Python, such a capability will not be
included in the standard library, however it is anticipated that
someone will create and publish a recipe for doing this.
Another type of customization is to change the way that built-in
types are formatted by overriding the 'format_field' method. (For
types are formatted by overriding the 'format_field' method. (For
non-built-in types, you can simply define a __format__ special
method on that type.) So for example, you could override the
method on that type.) So for example, you could override the
formatting of numbers to output scientific notation when needed.
@ -527,8 +548,7 @@ Error handling
There are two classes of exceptions which can occur during formatting:
exceptions generated by the formatter code itself, and exceptions
generated by user code (such as a field object's getattr function, or
the field_hook function).
generated by user code (such as a field object's 'getattr' function).
In general, exceptions generated by the formatter code itself are
of the "ValueError" variety -- there is an error in the actual "value"
@ -605,7 +625,7 @@ Alternate Syntax
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
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.
@ -615,6 +635,38 @@ Alternate Syntax
what .Net uses.
Alternate Feature Proposals
Restricting attribute access: An earlier version of the PEP
restricted the ability to access attributes beginning with a
leading underscore, for example "{0}._private". However, this
is a useful ability to have when debugging, so the feature
was dropped.
Some developers suggested that the ability to do 'getattr' and
'getitem' access should be dropped entirely. However, this
is in conflict with the needs of another set of developers who
strongly lobbied for the ability to pass in a large dict as a
single argument (without flattening it into individual keyword
arguments using the **kwargs syntax) and then have the format
string refer to dict entries individually.
There has also been suggestions to expand the set of expressions
that are allowed in a format string. However, this was seen
to go against the spirit of TOOWTDI, since the same effect can
be achieved in most cases by executing the same expression on
the parameter before it's passed in to the formatting function.
For cases where the format string is being use to do arbitrary
formatting in a data-rich environment, it's recommended to use
a templating engine specialized for this purpose, such as
Genshi [5] or Cheetah [6].
Many other features were considered and rejected because they
could easily be achieved by subclassing Formatter instead of
building the feature into the base implementation. This includes
alternate syntax, comments in format strings, and many others.
Security Considerations
Historically, string formatting has been a common source of
@ -622,43 +674,21 @@ Security Considerations
string templating system allows arbitrary expressions to be
embedded in format strings.
The typical scenario is one where the string data being processed
is coming from outside the application, perhaps from HTTP headers
or fields within a web form. An attacker could substitute their
own strings designed to cause havok.
The string formatting system outlined in this PEP is by no means
'secure', in the sense that no Python library module can, on its
own, guarantee security, especially given the open nature of
the Python language. Building a secure application requires a
secure approach to design.
What this PEP does attempt to do is make the job of designing a
secure application easier, by making it easier for a programmer
to reason about the possible consequences of a string formatting
operation. It does this by limiting those consequences to a smaller
and more easier understood subset.
For example, because it is possible in Python to override the
'getattr' operation of a type, the interpretation of a compound
replacement field such as "0.name" could potentially run
arbitrary code.
However, it is *extremely* rare for the mere retrieval of an
attribute to have side effects. Other operations which are more
likely to have side effects - such as method calls - are disallowed.
Thus, a programmer can be reasonably assured that no string
formatting operation will cause a state change in the program.
This assurance is not only useful in securing an application, but
in debugging it as well.
Similarly, the restriction on field names beginning with
underscores is intended to provide similar assurances about the
visibility of private data.
Of course, programmers would be well-advised to avoid using
any external data as format strings, and instead use that data
as the format arguments instead.
The best way to use string formatting in a way that does not
create potential security holes is to never use format strings
that come from an untrusted source.
Barring that, the next best approach is to insure that string
formatting has no side effects. Because of the open nature of
Python, it is impossible to guarantee that any non-trivial
operation has this property. What this PEP does is limit the
types of expressions in format strings to those in which visible
side effects are both rare and strongly discouraged by the
culture of Python developers. So for example, attribute access
is allowed because it would be considered pathological to write
code where the mere access of an attribute has visible side
effects (whether the code has *invisible* side effects - such
as creating a cache entry for faster lookup - is irrelevant.)
Sample Implementation
@ -692,6 +722,12 @@ References
[4] Composite Formatting - [.Net Framework Developer's Guide]
http://msdn.microsoft.com/library/en-us/cpguide/html/cpconcompositeformatting.asp?frame=true
[5] Genshi templating engine.
http://genshi.edgewall.org/
[5] Cheetah - The Python-Powered Template Engine.
http://www.cheetahtemplate.org/
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