In String module, Template Class allows us to create simplified syntax for output specification. The format uses placeholder names formed by $ with valid Python identifiers (alphanumeric characters and underscores). Surrounding the placeholder with braces allows it to be followed by more.
Source code:Lib/string.py
6.1.1. String constants¶
The constants defined in this module are:
The concatenation of the ascii_lowercase and ascii_uppercaseconstants described below. This value is not locale-dependent.
The lowercase letters 'abcdefghijklmnopqrstuvwxyz'. This value is notlocale-dependent and will not change.
The uppercase letters 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'. This value is notlocale-dependent and will not change.
The string '0123456789'.
The string '0123456789abcdefABCDEF'.
The string '01234567'.
String of ASCII characters which are considered punctuation charactersin the C locale.
String of ASCII characters which are considered printable. This is acombination of digits, ascii_letters, punctuation,and whitespace.
Python Template Engine
A string containing all ASCII characters that are considered whitespace.This includes the characters space, tab, linefeed, return, formfeed, andvertical tab.
6.1.2. String Formatting¶
The built-in string class provides the ability to do complex variablesubstitutions and value formatting via the format() method described inPEP 3101. The Formatter class in the string module allowsyou to create and customize your own string formatting behaviors using the sameimplementation as the built-in format() method.
The Formatter class has the following public methods:
format() is the primary API method. It takes a format string andan arbitrary set of positional and keyword arguments.format() is just a wrapper that calls vformat().
This function does the actual work of formatting. It is exposed as aseparate function for cases where you want to pass in a predefineddictionary of arguments, rather than unpacking and repacking thedictionary as individual arguments using the *args and **kwargssyntax. vformat() does the work of breaking up the format stringinto character data and replacement fields. It calls the variousmethods described below.
In addition, the Formatter defines a number of methods that areintended to be replaced by subclasses:
Loop over the format_string and return an iterable of tuples(literal_text, field_name, format_spec, conversion). This is usedby vformat() to break the string into either literal text, orreplacement fields.
The values in the tuple conceptually represent a span of literal textfollowed by a single replacement field. If there is no literal text(which can happen if two replacement fields occur consecutively), thenliteral_text will be a zero-length string. If there is no replacementfield, then the values of field_name, format_spec and conversionwill be None.
Given field_name as returned by parse() (see above), convert it toan object to be formatted. Returns a tuple (obj, used_key). The defaultversion takes strings of the form defined in PEP 3101, such as“0[name]” or “label.title”. args and kwargs are as passed in tovformat(). The return value used_key has the same meaning as thekey parameter to get_value().
Retrieve a given field value. The key argument will be either aninteger or a string. If it is an integer, it represents the index of thepositional argument in args; if it is a string, then it represents anamed argument in kwargs.
The args parameter is set to the list of positional arguments tovformat(), and the kwargs parameter is set to the dictionary ofkeyword arguments.
For compound field names, these functions are only called for the firstcomponent of the field name; Subsequent components are handled throughnormal attribute and indexing operations.
So for example, the field expression ‘0.name’ would causeget_value() to be called with a key argument of 0. The nameattribute will be looked up after get_value() returns by calling thebuilt-in getattr() function.
If the index or keyword refers to an item that does not exist, then anIndexError or KeyError should be raised.
Implement checking for unused arguments if desired. The arguments to thisfunction is the set of all argument keys that were actually referred to inthe format string (integers for positional arguments, and strings fornamed arguments), and a reference to the args and kwargs that waspassed to vformat. The set of unused args can be calculated from theseparameters. check_unused_args() is assumed to raise an exception ifthe check fails.
format_field() simply calls the global format() built-in. Themethod is provided so that subclasses can override it.
Converts the value (returned by get_field()) given a conversion type(as in the tuple returned by the parse() method). The defaultversion understands ‘s’ (str), ‘r’ (repr) and ‘a’ (ascii) conversiontypes.
6.1.3. Format String Syntax¶
The str.format() method and the Formatter class share the samesyntax for format strings (although in the case of Formatter,subclasses can define their own format string syntax).
Format strings contain “replacement fields” surrounded by curly braces {}.Anything that is not contained in braces is considered literal text, which iscopied unchanged to the output. If you need to include a brace character in theliteral 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 specifiesthe object whose value is to be formatted and insertedinto the output instead of the replacement field.The field_name is optionally followed by a conversion field, which ispreceded by an exclamation point '!', and a format_spec, which is precededby 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 akeyword. 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 stringare 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 arbitrarydictionary keys (e.g., the strings '10' or ':-]') within a format string.The arg_name can be followed by any number of index orattribute expressions. An expression of the form '.name' selects the namedattribute 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, so '{}{}' isequivalent to '{0}{1}'.
Some simple format string examples:
The conversion field causes a type coercion before formatting. Normally, thejob of formatting a value is done by the __format__() method of the valueitself. However, in some cases it is desirable to force a type to be formattedas a string, overriding its own definition of formatting. By converting thevalue to a string before calling __format__(), the normal formatting logicis bypassed.
Three conversion flags are currently supported: '!s' which calls str()on the value, '!r' which calls repr() and '!a' which callsascii().
Some examples:
The format_spec field contains a specification of how the value should bepresented, including such details as field width, alignment, padding, decimalprecision and so on. Each value type can define its own “formattingmini-language” or interpretation of the format_spec.
Most built-in types support a common formatting mini-language, which isdescribed in the next section.
A format_spec field can also include nested replacement fields within it.These nested replacement fields can contain only a field name; conversion flagsand format specifications are not allowed. The replacement fields within theformat_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 aformat string to define how individual values are presented (seeFormat String Syntax). They can also be passed directly to the built-informat() function. Each formattable type may define how the formatspecification 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 (') producesthe same result as if you had called str() on the value. Anon-empty format string typically modifies the result.
Python Template String Replace
The general form of a standard format specifier is:
If a valid align value is specified, it can be preceded by a fillcharacter that can be any character and defaults to a space if omitted.Note that it is not possible to use { and } as fill char whileusing the str.format() method; this limitation however doesn’taffect the format() function.
The meaning of the various alignment options is as follows:
| Option | Meaning |
|---|---|
| '<' | Forces the field to be left-aligned within the availablespace (this is the default for most objects). |
| '>' | Forces the field to be right-aligned within theavailable 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 fieldsin the form ‘+000000120’. This alignment option is onlyvalid for numeric types. |
| '^' | Forces the field to be centered within the availablespace. |
Note that unless a minimum field width is defined, the field width will alwaysbe the same size as the data to fill it, so that the alignment option has nomeaning in this case.
The sign option is only valid for number types, and can be one of thefollowing:
| Option | Meaning |
|---|---|
| '+' | indicates that a sign should be used for bothpositive as well as negative numbers. |
| '-' | indicates that a sign should be used only for negativenumbers (this is the default behavior). |
| space | indicates that a leading space should be used onpositive numbers, and a minus sign on negative numbers. |
The '#' option causes the “alternate form” to be used for theconversion. The alternate form is defined differently for differenttypes. This option is only valid for integer, float, complex andDecimal types. For integers, when binary, octal, or hexadecimal outputis used, this option adds the prefix respective '0b', '0o', or'0x' to the output value. For floats, complex and Decimal thealternate form causes the result of the conversion to always contain adecimal-point character, even if no digits follow it. Normally, adecimal-point character appears in the result of these conversionsonly 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 typeinstead.
Changed in version 3.1: Added the ',' option (see also PEP 378).
width is a decimal integer defining the minimum field width. If notspecified, then the field width will be determined by the content.
Preceding the width field by a zero ('0') character enablessign-aware zero-padding for numeric types. This is equivalent to a fillcharacter of '0' with an alignment type of '='.
The precision is a decimal number indicating how many digits should bedisplayed after the decimal point for a floating point value formatted with'f' and 'F', or before and after the decimal point for a floating pointvalue formatted with 'g' or 'G'. For non-number types the fieldindicates the maximum field size - in other words, how many characters will beused 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 andmay 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 correspondingunicode 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 usesthe current locale setting to insert the appropriatenumber separator characters. |
| None | The same as 'd'. |
In addition to the above presentation types, integers can be formattedwith the floating point presentation types listed below (except'n' and None). When doing so, float() is used to convert theinteger 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 scientificnotation using the letter ‘e’ to indicate the exponent.The default precision is 6. |
| 'E' | Exponent notation. Same as 'e' except it uses anupper case ‘E’ as the separator character. |
| 'f' | Fixed point. Displays the number as a fixed-pointnumber. The default precision is 6. |
| 'F' | Fixed point. Same as 'f', but converts nan toNAN and inf to INF. |
| 'g' | General format. For a given precision p>=1,this rounds the number to p significant digits andthen formats the result in either fixed-point formator in scientific notation, depending on its magnitude. The precise rules are as follows: suppose that theresult formatted with presentation type 'e' andprecision p-1 would have exponent exp. Thenif -4<=exp<p, the number is formattedwith presentation type 'f' and precisionp-1-exp. Otherwise, the number is formattedwith presentation type 'e' and precision p-1.In both cases insignificant trailing zeros are removedfrom the significand, and the decimal point is alsoremoved if there are no remaining digits following it. Positive and negative infinity, positive and negativezero, and nans, are formatted as inf, -inf,0, -0 and nan respectively, regardless ofthe precision. A precision of 0 is treated as equivalent to aprecision of 1. The default precision is 6. |
| 'G' | General format. Same as 'g' except switches to'E' if the number gets too large. Therepresentations of infinity and NaN are uppercased, too. |
| 'n' | Number. This is the same as 'g', except that it usesthe current locale setting to insert the appropriatenumber separator characters. |
| '%' | Percentage. Multiplies the number by 100 and displaysin 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 representthe particular value. The overall effect is to match theoutput of str() as altered by the other formatmodifiers. |
6.1.3.2. Format examples¶
This section contains examples of the new format syntax and comparison withthe old %-formatting.
In most of the cases the syntax is similar to the old %-formatting, with theaddition of the {} and with : used instead of %.For example, '%03.2f' can be translated to '{:03.2f}'.
The new format syntax also supports new and different options, shown in thefollow examples.
Accessing arguments by position:
Accessing arguments by name:
Accessing arguments’ attributes:
Accessing arguments’ items:
Replacing %s and %r:
Aligning the text and specifying a width:
Replacing %+f, %-f, and %f and specifying a sign:
Replacing %x and %o and converting the value to different bases:
Using the comma as a thousands separator:
Expressing a percentage:
Using type-specific formatting:
Nesting arguments and more complex examples:
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 anycase-insensitive ASCII alphanumeric string (including underscores) thatstarts with an underscore or ASCII letter. The first non-identifiercharacter after the $ character terminates this placeholderspecification.
- ${identifier} is equivalent to $identifier. It is required whenvalid identifier characters follow the placeholder but are not part of theplaceholder, such as '${noun}ification'.
Any other appearance of $ in the string will result in a ValueErrorbeing raised.
The string module provides a Template class that implementsthese rules. The methods of Template are:
The constructor takes a single argument which is the template string.
Performs the template substitution, returning a new string. mapping isany dictionary-like object with keys that match the placeholders in thetemplate. Alternatively, you can provide keyword arguments, where thekeywords are the placeholders. When both mapping and kwds are givenand there are duplicates, the placeholders from kwds take precedence.
Like substitute(), except that if placeholders are missing frommapping and kwds, instead of raising a KeyError exception, theoriginal placeholder will appear in the resulting string intact. Also,unlike with substitute(), any other appearances of the $ willsimply 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 ofraising an exception. In another sense, safe_substitute() may beanything other than safe, since it will silently ignore malformedtemplates containing dangling delimiters, unmatched braces, orplaceholders that are not valid Python identifiers.
Template instances also provide one public data attribute:
This is the object passed to the constructor’s template argument. Ingeneral, you shouldn’t change it, but read-only access is not enforced.
Here is an example of how to use a Template:
Advanced usage: you can derive subclasses of Template to customize theplaceholder syntax, delimiter character, or the entire regular expression usedto parse template strings. To do this, you can override these class attributes:
delimiter – This is the literal string describing a placeholder introducingdelimiter. The default value is $. Note that this should not be aregular expression, as the implementation will call re.escape() on thisstring as needed.
idpattern – This is the regular expression describing the pattern fornon-braced placeholders (the braces will be added automatically asappropriate). The default value is the regular expression[_a-z][_a-z0-9]*.
flags – The regular expression flags that will be applied when compilingthe regular expression used for recognizing substitutions. The default valueis re.IGNORECASE. Note that re.VERBOSE will always be added to theflags, so custom idpatterns must follow conventions for verbose regularexpressions.
New in version 3.2.
Alternatively, you can provide the entire regular expression pattern byoverriding the class attribute pattern. If you do this, the value must be aregular expression object with four named capturing groups. The capturinggroups correspond to the rules given above, along with the invalid placeholderrule:
- escaped – This group matches the escape sequence, e.g. $$, in thedefault pattern.
- named – This group matches the unbraced placeholder name; it should notinclude the delimiter in capturing group.
- braced – This group matches the brace enclosed placeholder name; it shouldnot include either the delimiter or braces in the capturing group.
- invalid – This group matches any other delimiter pattern (usually a singledelimiter), and it should appear last in the regular expression.
6.1.5. Helper functions¶
Split the argument into words using str.split(), capitalize each wordusing str.capitalize(), and join the capitalized words usingstr.join(). If the optional second argument sep is absentor None, runs of whitespace characters are replaced by a single spaceand leading and trailing whitespace are removed, otherwise sep is used tosplit and join the words.
I was wondering if there is a advantage of using template strings instead of the new advanced string formatting?
dreftymac4 Answers
Templates are meant to be simpler than the the usual string formatting, at the cost of expressiveness. The rationale of PEP 292 compares templates to Python's %-style string formatting:
Python currently supports a string substitution syntax based on C's printf() '%' formatting character. While quite rich, %-formatting codes are also error prone, even for experienced Python programmers. A common mistake is to leave off the trailing format character, e.g. the s in %(name)s.
In addition, the rules for what can follow a % sign are fairly complex, while the usual application rarely needs such complexity. Most scripts need to do some string interpolation, but most of those use simple stringification' formats, i.e.%sor%(name)s` This form should be made simpler and less error prone.
While the new .format() improved the situation, it's still true that the format string syntax is rather complex, so the rationale still has its points.
For what it's worth, Template substitution from a dict appears to be 4 to 10 times slower than format substitution, depending on the length of the template. Here's a quick comparison I ran under OS X on a 2.3 GHz core i7 with Python 3.5.
The worst case I tested was about 10 times slower for a 13 character string. The best case I tested was about 4 times slower for a 71000 character string.
One key advantage of string templates is that you can substitute only some of the placeholders using the safe_substitutemethod. Normal format strings will raise an error if a placeholder is not passed a value. For example:
raises:
But:
Produces:
Note that the returned value is a string, not a Template; if you want to substitute the $last you'll need to create a new Template object from that string.
Its primarily a matter of syntax preference, which usually boils down to a laziness/verbosity tradeoff and familiarity/habits with existing string template systems. In this case template strings are more lazy/simple/quick to write, while .format() is more verbose and feature-full.
Programmers used to the PHP language or the Jinja family of template systems may prefer template strings. Using '%s' positional style tuple substitution might appeal to those who use printf-like string formatting or want something quick. .format() has a few more features, but unless you need something specific that only .format() provides, there is nothing wrong with using any existing scheme.
The only thing to be aware of is that named string templates are more flexible and require less maintenance than order-dependent ones. Other than that it all comes down to either personal preference or the coding standard of the project you are working on;