[Python-3000] PEP: str(container) should call str(item), not repr(item)
phd at phd.pp.ru
Thu May 29 21:21:57 CEST 2008
Hello. A draft for a discussion.
Title: str(container) should call str(item), not repr(item)
Author: Oleg Broytmann <phd at phd.pp.ru>,
Jim Jewett <jimjjewett at gmail.com>
Discussions-To: python-3000 at python.org
Type: Standards Track
This document discusses the advantages and disadvantages of the
current implementation of str(container). It also discusses the
pros and cons of a different approach - to call str(item) instead
Currently str(container) calls repr on items. Arguments for it:
-- containers refuse to guess what the user wants to see on
str(container) - surroundings, delimiters, and so on;
-- repr(item) usually displays type information - apostrophes
around strings, class names, etc.
-- it's illogical; str() is expected to call __str__ if it exists,
-- there is no standard way to print a container's content calling
items' __str__, that's inconvenient in cases where __str__ and
__repr__ return different results;
-- repr(item) sometimes do wrong things (hex-escapes non-ascii
This PEP proposes to change how str(container) works. It is
proposed to mimic how repr(container) works except one detail
- call str on items instead of repr. This allows a user to choose
what results she want to get - from item.__repr__ or item.__str__.
Most container types (tuples, lists, dicts, sets, etc.) do not
implement __str__ method, so str(container) calls
container.__repr__, and container.__repr__, once called, forgets
it is called from str and always calls repr on the container's
This behaviour has advantages and disadvantages. One advantage is
that most items are represented with type information - strings
are surrounded by apostrophes, instances may have both class name
and instance data:
>>> print([42, '42'])
>>> print([Decimal('42'), datetime.now()])
[Decimal("42"), datetime.datetime(2008, 5, 27, 19, 57, 43, 485028)]
The disadvantage is that __repr__ often returns technical data
(like '<object at address>') or unreadable string (hex-encoded
string if the input is non-ascii string):
One of the motivations for PEP 3138 is that neither repr nor str
will allow the sensible printing of dicts whose keys are non-ascii
text strings. Now that unicode identifiers are allowed, it
includes Python's own attribute dicts. This also includes JSON
serialization (and caused some hoops for the json lib).
PEP 3138 proposes to fix this by breaking the "repr is safe ASCII"
invariant, and changing the way repr (which is used for
persistence) outputs some objects, with system-dependent failures.
Changing how str(container) works would allow easy debugging in
the normal case, and retrain the safety of ASCII-only for the
machine-readable case. The only downside is that str(x) and
repr(x) would more often be different -- but only in those cases
where the current almost-the-same version is insufficient.
It also seems illogical that str(container) calls repr on items
instead of str. It's only logical to expect following code
test = Test()
where it actually prints
Especially it is illogical to see that print in Python 2 uses str
if it is called on what seems to be a tuple:
>>> print Decimal('42'), datetime.now()
42 2008-05-27 20:16:22.534285
where on an actual tuple it prints
>>> print((Decimal('42'), datetime.now()))
(Decimal("42"), datetime.datetime(2008, 5, 27, 20, 16, 27, 937911))
A different approach - call str(item)
For example, with numbers it is often only the value that people
>>> print Decimal('3')
But putting the value in a list forces users to read the type
information, exactly as if repr had been called for the benefit of
>>> print [Decimal('3')]
After this change, the type information would not clutter the str
>>> print "%s".format([Decimal('3')])
>>> str([Decimal('3')]) # ==
But it would still be available if desired:
>>> print "%r".format([Decimal('3')])
>>> repr([Decimal('3')]) # ==
There is a number of strategies to fix the problem. The most
radical is to change __repr__ so it accepts a new parameter (flag)
"called from str, so call str on items, not repr". The
drawback of the proposal is that every __repr__ implementation
must be changed. Introspection could help a bit (inspect __repr__
before calling if it accepts 2 or 3 parameters), but introspection
doesn't work on classes written in C, like all builtin containers.
Less radical proposal is to implement __str__ methods for builtin
container types. The obvious drawback is a duplication of effort
- all those __str__ and __repr__ implementations are only differ
in one small detail - if they call str or repr on items.
The most conservative proposal is not to change str at all but
to allow developers to implement their own application- or
library-specific pretty-printers. The drawback is again
a multiplication of effort and proliferation of many small
specific container-traversal algorithms.
In those cases where type information is more important than
usual, it will still be possible to get the current results by
calling repr explicitly.
This document has been placed in the public domain.
Oleg Broytmann http://phd.pp.ru/ phd at phd.pp.ru
Programmers don't die, they just GOSUB without RETURN.
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