New submission from Tobia Conforto:
The format specification mini-language (format_spec) supported by format() and
str.format() is a feature that allows passing short options to the classes of
the values being formatted, to drive their string representation (__format__
method)
The most common operation done to sequences (lists, tuples, sets...) during
conversion to string is arguably the string join operation, possibly coupled
with a "nested" string formatting of the sequence items.
I propose the addition of a custom format_spec for sequences, that allows to
easily specify a string for the join operation and optionally a nested
format_spec to be passed along to format the sequence items.
Here is the proposed addition:
seq_format_spec ::= join_string [":" item_format_spec] | format_spec
join_string ::= '"' join_string_char* '"' | "'" join_string_char* "'"
join_string_char ::= <any character except "{", "}", newline, or the quote>
item_format_spec ::= format_spec
In words, if the format_spec for a sequence starts with a single or double
quote, it will be interpreted as a join operation, optionally followed by
another colon and the format_spec for the sequnce items.
If the format_spec does not start with ' or ", of if the quote is not balanced
(does not appear again in the format_spec), then it's assumed to be a generic
format string and the implementation would call super(). This ensures backwards
compatibility with existing code that may be using object's __format__
implementation on various sequence objects.
Please note I'm NOT proposing a change in the language or in the implementation
of format() and str.format(). This is just the addition of a __format__ method
to lists, tuples, sets and other sequence classes. The choice of whether to do
that in all those sequence classes or as an addition to object's __format__ is
an implementation detail.
Examples:
Basic usage: either {0:", "} or {0:', '} when used in a format operation will
do this: ", ".join(str(x) for x in argument_0) in a more compact, possibly more
efficient, and arguably easier to read syntax.
Nested (regular) format_spec: {0:", ":.1f} will join a list of floats using ",
" as the separator and .1f as the format_spec for each float.
Nested join format_spec: {0:"\n":", "} will join a list of lists, using "\n" as
the outer separator and ", " as the inner separator. This could go on
indefinitely (but will rarely need to do so.)
I do not have a patch ready, but I can work on it and submit it for evaluation,
if this enhancement is accepted.
----------
messages: 182376
nosy: tobia
priority: normal
severity: normal
status: open
title: format_spec for sequence joining
type: enhancement
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Python tracker <[email protected]>
<http://bugs.python.org/issue17236>
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