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 ofstr(container).It also discusses the pros and cons of a different approach - to callstr(item)instead ofrepr(item).

Motivation

Currentlystr(container)callsrepron 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 howstr(container)works. It is proposed to mimic howrepr(container)works except one detail - call stron items instead ofrepr.This allows a user to choose what results she want to get - fromitem.__repr__oritem.__str__.

Current situation

Most container types (tuples, lists, dicts, sets, etc.) do not implement__str__method, sostr(container)calls container.__repr__,andcontainer.__repr__,once called, forgets it is called fromstrand always callsrepron 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 ‘<objectataddress>’) or unreadable string (hex-encoded string if the input is non-ASCII string):

>>>print(['тест'])
['\xd4\xc5\xd3\xd4']

One of the motivations forPEP 3138is that neitherreprnorstr 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 3138proposes to fix this by breaking the “repr is safe ASCII” invariant, and changing the wayrepr(which is used for persistence) outputs some objects, with system-dependent failures.

Changing howstr(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 thatstr(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 thatstr(container)callsrepron items instead ofstr.It’s only logical to expect following code:

classTest:
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 usesstr if it is called on what seems to be a tuple:

>>>printDecimal('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 - callstr(item)

For example, with numbers it is often only the value that people care about.

>>>printDecimal('3')
3

But putting the value in a list forces users to read the type information, exactly as ifreprhad been called for the benefit of a machine:

>>>print[Decimal('3')]
[Decimal( "3" )]

After this change, the type information would not clutter thestr 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 fromstr,so callstron items, notrepr”.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 callstrorrepron 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 callingreprexplicitly.

References

Copyright

This document has been placed in the public domain.