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

[1]

Guido van Rossum, PEP: str(container) should call str(item), not repr(item) https://mail.python.org/pipermail/python-3000/2008-May/013876.html

Copyright

This document has been placed in the public domain.