[Python-bugs-list] [ python-Bugs-575536 ] Concatenating a tuple to a list

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Bugs item #575536, was opened at 2002-06-29 19:18
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Category: Python Interpreter Core
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Dan Grassi (dgrassi)
Assigned to: Tim Peters (tim_one)
Summary: Concatenating a tuple to a list

Initial Comment:
"a=a+b" is not the same as "a+=b" if a is a list and b is a tuple.  See the code below.

This has been tested on 2.2.1 MacPython, 2.2 MachoPython and 2.2 Python on an Alpha.

Augmented assignment (+=) with a list on the LHS allows a tuple on the RHS.  Standard assignment does not.

This seems intuitively wrong.

--- test code ---
a=[1]
b=(2,)

print 1,type(a), a
print 1,type(b), b
c=a+b

print 2,type(a), a
print 2,type(b), b
a=a+b

print 3,type(a), a
print 3,type(b), b
a+=b

print 4,type(a), a
print 4,type(b), b

Below is the execution results of the above:

>>>
>>> a=[1]
>>> b=(2,)
>>>
>>> print 1,type(a), a
1 <type 'list'> [1]
>>> print 1,type(b), b
1 <type 'tuple'> (2,)
>>> c=a+b
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: can only concatenate list (not "tuple") to list
>>>
>>> print 2,type(a), a
2 <type 'list'> [1]
>>> print 2,type(b), b
2 <type 'tuple'> (2,)
>>> a=a+b
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: can only concatenate list (not "tuple") to list
>>>
>>> print 3,type(a), a
3 <type 'list'> [1]
>>> print 3,type(b), b
3 <type 'tuple'> (2,)
>>> a+=b
>>>
>>> print 4,type(a), a
4 <type 'list'> [1, 2]
>>> print 4,type(b), b
4 <type 'tuple'> (2,)
>>>


----------------------------------------------------------------------

>Comment By: Raymond Hettinger (rhettinger)
Date: 2002-06-30 14:00

Message:
Logged In: YES 
user_id=80475

Agreed. My first instinct was to restrict list_inplace_concat
() rather than liberalize list_concat(); however, it is already 
out in the wild and code may be relying on it.

One change or the other should be made since the docs 
promise that 'a op= b'  is equivalent to 'a = a op b' unless 
specifically overriden or when get/set attr is used to 
access 'a'.

Your example highlights the weirdness that can ensue; 
however, I think we already started down that road when 
everything was made iterable and, therefore, substitutable 
into weird combinations:

>>> zip([1], 'hallo', {1:2})
[(1, 'h', 1)]

----------------------------------------------------------------------

Comment By: Martin v. Löwis (loewis)
Date: 2002-06-30 13:43

Message:
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user_id=21627

I'm not so sure that the current += behaviour is desirable.
It means that

a=[1]
a+="hallo"
a+={1:2}

is possible, which should be a TypeError, IMO.

----------------------------------------------------------------------

Comment By: Raymond Hettinger (rhettinger)
Date: 2002-06-30 11:35

Message:
Logged In: YES 
user_id=80475

Confirmed.  The behavior of list_inplace_concat diverged 
from list_concat.

See attached patch. Recommend applying to Py2.3 only 
since inconsistency isn't a bug.

----------------------------------------------------------------------

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