6.1.3. Format String Syntax
The str.format() method and the Formatter class share the same
syntax for format strings (although in the case of Formatter,
subclasses can define their own format string syntax). The syntax is
related to that of formatted string literals, but
there are differences.
Format strings contain “replacement fields” surrounded by curly braces {}.
Anything that is not contained in braces is considered literal text, which is
copied unchanged to the output. If you need to include a brace character in the
literal text, it can be escaped by doubling: {{ and }}.
The grammar for a replacement field is as follows:
In less formal terms, the replacement field can start with a field_name that specifies
the object whose value is to be formatted and inserted
into the output instead of the replacement field.
The field_name is optionally followed by a conversion field, which is
preceded by an exclamation point '!', and a format_spec, which is preceded
by a colon ':'. These specify a non-default format for the replacement value.
See also the Format Specification Mini-Language section.
The field_name itself begins with an arg_name that is either a number or a
keyword. If it’s a number, it refers to a positional argument, and if it’s a keyword,
it refers to a named keyword argument. If the numerical arg_names in a format string
are 0, 1, 2, … in sequence, they can all be omitted (not just some)
and the numbers 0, 1, 2, … will be automatically inserted in that order.
Because arg_name is not quote-delimited, it is not possible to specify arbitrary
dictionary keys (e.g., the strings '10' or ':-]') within a format string.
The arg_name can be followed by any number of index or
attribute expressions. An expression of the form '.name' selects the named
attribute using getattr(), while an expression of the form '[index]'
does an index lookup using __getitem__().
Changed in version 3.1: The positional argument specifiers can be omitted for str.format(),
so '{} {}'.format(a, b) is equivalent to '{0} {1}'.format(a, b).
Changed in version 3.4: The positional argument specifiers can be omitted for Formatter.
Some simple format string examples:
"First, thou shalt count to {0}" # References first positional argument
"Bring me a {}" # Implicitly references the first positional argument
"From {} to {}" # Same as "From {0} to {1}"
"My quest is {name}" # References keyword argument 'name'
"Weight in tons {0.weight}" # 'weight' attribute of first positional arg
"Units destroyed: {players[0]}" # First element of keyword argument 'players'.
The conversion field causes a type coercion before formatting. Normally, the
job of formatting a value is done by the __format__() method of the value
itself. However, in some cases it is desirable to force a type to be formatted
as a string, overriding its own definition of formatting. By converting the
value to a string before calling __format__(), the normal formatting logic
is bypassed.
Three conversion flags are currently supported: '!s' which calls str()
on the value, '!r' which calls repr() and '!a' which calls
ascii().
Some examples:
"Harold's a clever {0!s}" # Calls str() on the argument first
"Bring out the holy {name!r}" # Calls repr() on the argument first
"More {!a}" # Calls ascii() on the argument first
The format_spec field contains a specification of how the value should be
presented, including such details as field width, alignment, padding, decimal
precision and so on. Each value type can define its own “formatting
mini-language” or interpretation of the format_spec.
Most built-in types support a common formatting mini-language, which is
described in the next section.
A format_spec field can also include nested replacement fields within it.
These nested replacement fields may contain a field name, conversion flag
and format specification, but deeper nesting is
not allowed. The replacement fields within the
format_spec are substituted before the format_spec string is interpreted.
This allows the formatting of a value to be dynamically specified.
See the Format examples section for some examples.
6.1.3.1. Format Specification Mini-Language
“Format specifications” are used within replacement fields contained within a
format string to define how individual values are presented (see
Format String Syntax and Formatted string literals).
They can also be passed directly to the built-in
format() function. Each formattable type may define how the format
specification is to be interpreted.
Most built-in types implement the following options for format specifications,
although some of the formatting options are only supported by the numeric types.
A general convention is that an empty format string ("") produces
the same result as if you had called str() on the value. A
non-empty format string typically modifies the result.
The general form of a standard format specifier is:
format_spec ::= [[fill]align][sign][#][0][width][grouping_option][.precision][type]
fill ::= <any character>
align ::= "<" | ">" | "=" | "^"
sign ::= "+" | "-" | " "
width ::= digit+
grouping_option ::= "_" | ","
precision ::= digit+
type ::= "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"
If a valid align value is specified, it can be preceded by a fill
character that can be any character and defaults to a space if omitted.
It is not possible to use a literal curly brace (“{” or “}”) as
the fill character in a formatted string literal or when using the str.format()
method. However, it is possible to insert a curly brace
with a nested replacement field. This limitation doesn’t
affect the format() function.
The meaning of the various alignment options is as follows:
| Option |
Meaning |
'<' |
Forces the field to be left-aligned within the available
space (this is the default for most objects). |
'>' |
Forces the field to be right-aligned within the
available space (this is the default for numbers). |
'=' |
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’. This alignment option is only
valid for numeric types. It becomes the default when ‘0’
immediately precedes the field width. |
'^' |
Forces the field to be centered within the available
space. |
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 sign option is only valid for number types, and can be one of the
following:
| Option |
Meaning |
'+' |
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 behavior). |
| space |
indicates that a leading space should be used on
positive numbers, and a minus sign on negative numbers. |
The '#' option causes the “alternate form” to be used for the
conversion. The alternate form is defined differently for different
types. This option is only valid for integer, float, complex and
Decimal types. For integers, when binary, octal, or hexadecimal output
is used, this option adds the prefix respective '0b', '0o', or
'0x' to the output value. For floats, complex and Decimal the
alternate form causes the result of the conversion to always contain a
decimal-point character, even if no digits follow it. Normally, a
decimal-point character appears in the result of these conversions
only if a digit follows it. In addition, for 'g' and 'G'
conversions, trailing zeros are not removed from the result.
The ',' option signals the use of a comma for a thousands separator.
For a locale aware separator, use the 'n' integer presentation type
instead.
Changed in version 3.1: Added the ',' option (see also PEP 378).
The '_' option signals the use of an underscore for a thousands
separator for floating point presentation types and for integer
presentation type 'd'. For integer presentation types 'b',
'o', 'x', and 'X', underscores will be inserted every 4
digits. For other presentation types, specifying this option is an
error.
Changed in version 3.6: Added the '_' option (see also PEP 515).
width is a decimal integer defining the minimum field width. If not
specified, then the field width will be determined by the content.
When no explicit alignment is given, preceding the width field by a zero
('0') character enables
sign-aware zero-padding for numeric types. This is equivalent to a fill
character of '0' with an alignment type of '='.
The precision is a decimal number indicating how many digits should be
displayed after the decimal point for a floating point value formatted with
'f' and 'F', or before and after the decimal point for a floating point
value formatted with 'g' or 'G'. For non-number types the field
indicates the maximum field size - in other words, how many characters will be
used from the field content. The precision is not allowed for integer values.
Finally, the type determines how the data should be presented.
The available string presentation types are:
| Type |
Meaning |
's' |
String format. This is the default type for strings and
may be omitted. |
| None |
The same as 's'. |
The available integer presentation types are:
| Type |
Meaning |
'b' |
Binary format. 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. |
'n' |
Number. This is the same as 'd', except that it uses
the current locale setting to insert the appropriate
number separator characters. |
| None |
The same as 'd'. |
In addition to the above presentation types, integers can be formatted
with the floating point presentation types listed below (except
'n' and None). When doing so, float() is used to convert the
integer to a floating point number before formatting.
The available presentation types for floating point and decimal values are:
| Type |
Meaning |
'e' |
Exponent notation. Prints the number in scientific
notation using the letter ‘e’ to indicate the exponent.
The default precision is 6. |
'E' |
Exponent notation. Same as 'e' except it uses an
upper case ‘E’ as the separator character. |
'f' |
Fixed-point notation. Displays the number as a
fixed-point number. The default precision is 6. |
'F' |
Fixed-point notation. Same as 'f', but converts
nan to NAN and inf to INF. |
'g' |
General format. For a given precision p >= 1,
this rounds the number to p significant digits and
then formats the result in either fixed-point format
or in scientific notation, depending on its magnitude.
The precise rules are as follows: suppose that the
result formatted with presentation type 'e' and
precision p-1 would have exponent exp. Then
if -4 <= exp < p, the number is formatted
with presentation type 'f' and precision
p-1-exp. Otherwise, the number is formatted
with presentation type 'e' and precision p-1.
In both cases insignificant trailing zeros are removed
from the significand, and the decimal point is also
removed if there are no remaining digits following it.
Positive and negative infinity, positive and negative
zero, and nans, are formatted as inf, -inf,
0, -0 and nan respectively, regardless of
the precision.
A precision of 0 is treated as equivalent to a
precision of 1. The default precision is 6.
|
'G' |
General format. Same as 'g' except switches to
'E' if the number gets too large. The
representations of infinity and NaN are uppercased, too. |
'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. |
| None |
Similar to 'g', except that fixed-point notation,
when used, has at least one digit past the decimal point.
The default precision is as high as needed to represent
the particular value. The overall effect is to match the
output of str() as altered by the other format
modifiers. |
6.1.4. Template strings
Templates provide simpler string substitutions as described in PEP 292.
Instead of the normal %-based substitutions, Templates support $-based substitutions, using the following rules:
$$ is an escape; it is replaced with a single $.
$identifier names a substitution placeholder matching a mapping key of
"identifier". By default, "identifier" is restricted to any
case-insensitive ASCII alphanumeric string (including underscores) that
starts with an underscore or ASCII letter. The first non-identifier
character after the $ character terminates this placeholder
specification.
${identifier} is equivalent to $identifier. It is required when
valid identifier characters follow the placeholder but are not part of the
placeholder, such as "${noun}ification".
Any other appearance of $ in the string will result in a ValueError
being raised.
The string module provides a Template class that implements
these rules. The methods of Template are:
-
class
string.Template(template)
The constructor takes a single argument which is the template string.
-
substitute(mapping, **kwds)
Performs the template substitution, returning a new string. mapping is
any dictionary-like object with keys that match the placeholders in the
template. Alternatively, you can provide keyword arguments, where the
keywords are the placeholders. When both mapping and kwds are given
and there are duplicates, the placeholders from kwds take precedence.
-
safe_substitute(mapping, **kwds)
Like substitute(), except that if placeholders are missing from
mapping and kwds, instead of raising a KeyError exception, the
original placeholder will appear in the resulting string intact. Also,
unlike with substitute(), any other appearances of the $ will
simply return $ instead of raising ValueError.
While other exceptions may still occur, this method is called “safe”
because substitutions always tries to return a usable string instead of
raising an exception. In another sense, safe_substitute() may be
anything other than safe, since it will silently ignore malformed
templates containing dangling delimiters, unmatched braces, or
placeholders that are not valid Python identifiers.
Template instances also provide one public data attribute:
-
template
This is the object passed to the constructor’s template argument. In
general, you shouldn’t change it, but read-only access is not enforced.
Here is an example of how to use a Template:
>>> from string import Template
>>> s = Template('$who likes $what')
>>> s.substitute(who='tim', what='kung pao')
'tim likes kung pao'
>>> d = dict(who='tim')
>>> Template('Give $who $100').substitute(d)
Traceback (most recent call last):
...
ValueError: Invalid placeholder in string: line 1, col 11
>>> Template('$who likes $what').substitute(d)
Traceback (most recent call last):
...
KeyError: 'what'
>>> Template('$who likes $what').safe_substitute(d)
'tim likes $what'
Advanced usage: you can derive subclasses of Template to customize the
placeholder syntax, delimiter character, or the entire regular expression used
to parse template strings. To do this, you can override these class attributes:
delimiter – This is the literal string describing a placeholder introducing
delimiter. The default value is $. Note that this should not be a
regular expression, as the implementation will call re.escape() on this
string as needed.
idpattern – This is the regular expression describing the pattern for
non-braced placeholders (the braces will be added automatically as
appropriate). The default value is the regular expression
(?-i:[_a-zA-Z][_a-zA-Z0-9]*).
Note
Since default flags is re.IGNORECASE, pattern [a-z] can match
with some non-ASCII characters. That’s why we use local -i flag here.
While flags is kept to re.IGNORECASE for backward compatibility,
you can override it to 0 or re.IGNORECASE | re.ASCII when
subclassing.
flags – The regular expression flags that will be applied when compiling
the regular expression used for recognizing substitutions. The default value
is re.IGNORECASE. Note that re.VERBOSE will always be added to the
flags, so custom idpatterns must follow conventions for verbose regular
expressions.
Alternatively, you can provide the entire regular expression pattern by
overriding the class attribute pattern. If you do this, the value must be a
regular expression object with four named capturing groups. The capturing
groups correspond to the rules given above, along with the invalid placeholder
rule:
- escaped – This group matches the escape sequence, e.g.
$$, in the
default pattern.
- named – This group matches the unbraced placeholder name; it should not
include the delimiter in capturing group.
- braced – This group matches the brace enclosed placeholder name; it should
not include either the delimiter or braces in the capturing group.
- invalid – This group matches any other delimiter pattern (usually a single
delimiter), and it should appear last in the regular expression.