PEP 3140 – str(container) should call str(item), not repr(item)
- Author:
- Oleg Broytman <phd at phdru.name>, Jim J. Jewett <jimjjewett at gmail.com>
- Discussions-To:
- Python-3000 list
- Status:
- Rejected
- Type:
- Standards Track
- Created:
- 27-May-2008
- Post-History:
- 28-May-2008
Rejection
Guido said this would cause too much disturbance too close to beta. See [1].
Abstract
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
of repr(item)
.
Motivation
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.
Arguments against:
- it’s illogical;
str()
is expected to call__str__
if it exists, not__repr__
; - 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 strings, e.g.)
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__
.
Current situation
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
items.
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'])
[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):
>>> print(['тест'])
['\xd4\xc5\xd3\xd4']
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 retain 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:
class Test:
def __str__(self):
return "STR"
def __repr__(self):
return "REPR"
test = Test()
print(test)
print(repr(test))
print([test])
print(str([test]))
to print:
STR
REPR
[STR]
[STR]
where it actually prints:
STR
REPR
[REPR]
[REPR]
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 care about.
>>> print Decimal('3')
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
a machine:
>>> print [Decimal('3')]
[Decimal("3")]
After this change, the type information would not clutter the str
output:
>>> print "%s".format([Decimal('3')])
[3]
>>> str([Decimal('3')]) # ==
[3]
But it would still be available if desired:
>>> print "%r".format([Decimal('3')])
[Decimal('3')]
>>> repr([Decimal('3')]) # ==
[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 built-in containers.
Less radical proposal is to implement __str__
methods for built-in
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.
Backward compatibility
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.
References
Copyright
This document has been placed in the public domain.
Source: https://github.com/python/peps/blob/main/pep-3140.txt
Last modified: 2022-04-20 09:53:08 GMT