Abstract

This document proposes a string interpolation feature for Python to allow easier string formatting. The suggested syntax change is the introduction of a ‘$’ prefix that triggers the special interpretation of the ‘$’ character within a string, in a manner reminiscent to the variable interpolation found in Unix shells, awk, Perl, or Tcl.

Copyright

This document is in the public domain.

Specification

Strings may be preceded with a ‘$’ prefix that comes before the leading single or double quotation mark (or triplet) and before any of the other string prefixes (‘r’ or ‘u’). Such a string is processed for interpolation after the normal interpretation of backslash-escapes in its contents. The processing occurs just before the string is pushed onto the value stack, each time the string is pushed. In short, Python behaves exactly as if ‘$’ were a unary operator applied to the string. The operation performed is as follows:

The string is scanned from start to end for the ‘$’ character (\x24 in 8-bit strings or \u0024 in Unicode strings). If there are no ‘$’ characters present, the string is returned unchanged.

Any ‘$’ found in the string, followed by one of the two kinds of expressions described below, is replaced with the value of the expression as evaluated in the current namespaces. The value is converted with str() if the containing string is an 8-bit string, or with unicode() if it is a Unicode string.

1. A Python identifier optionally followed by any number of trailers, where a trailer consists of: - a dot and an identifier, - an expression enclosed in square brackets, or - an argument list enclosed in parentheses (This is exactly the pattern expressed in the Python grammar by “NAME trailer*”, using the definitions in Grammar/Grammar.)
2. Any complete Python expression enclosed in curly braces.

Two dollar-signs (“$$”) are replaced with a single “$”.

Examples

Here is an example of an interactive session exhibiting the expected behaviour of this feature.

>>> a, b = 5, 6
>>> print $'a = $a, b = $b'
a = 5, b = 6
>>> $u'uni${a}ode'
u'uni5ode'
>>> print $'\$a'
5
>>> print $r'\$a'
\5
>>> print $'$$$a.$b'
$5.6
>>> print $'a + b = ${a + b}'
a + b = 11
>>> import sys
>>> print $'References to $a: $sys.getrefcount(a)'
References to 5: 15
>>> print $"sys = $sys, sys = $sys.modules['sys']"
sys = <module 'sys' (built-in)>, sys = <module 'sys' (built-in)>
>>> print $'BDFL = $sys.copyright.split()[4].upper()'
BDFL = GUIDO

Discussion

‘$’ is chosen as the interpolation character within the string for the sake of familiarity, since it is already used for this purpose in many other languages and contexts.

It is then natural to choose ‘$’ as a prefix, since it is a mnemonic for the interpolation character.

Trailers are permitted to give this interpolation mechanism even more power than the interpolation available in most other languages, while the expression to be interpolated remains clearly visible and free of curly braces.

‘$’ works like an operator and could be implemented as an operator, but that prevents the compile-time optimization and presents security issues. So, it is only allowed as a string prefix.

Security Issues

“$” has the power to eval, but only to eval a literal. As described here (a string prefix rather than an operator), it introduces no new security issues since the expressions to be evaluated must be literally present in the code.

Implementation

The Itpl module at [1] provides a prototype of this feature. It uses the tokenize module to find the end of an expression to be interpolated, then calls eval() on the expression each time a value is needed. In the prototype, the expression is parsed and compiled again each time it is evaluated.

As an optimization, interpolated strings could be compiled directly into the corresponding bytecode; that is,

$'a = $a, b = $b'

could be compiled as though it were the expression

('a = ' + str(a) + ', b = ' + str(b))

so that it only needs to be compiled once.

References