Compounded expressions with lambda functions are evaluated incorrectly

[mvyskocil]

BPO	6740
Nosy	@ericvsmith

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2009-08-20.16:12:25.830>
created_at = <Date 2009-08-20.10:27:16.225>
labels = ['type-bug', 'invalid']
title = 'Compounded expressions with lambda functions are evaluated incorrectly'
updated_at = <Date 2009-08-20.16:12:25.819>
user = 'https://bugs.python.org/mvyskocil'

bugs.python.org fields:

activity = <Date 2009-08-20.16:12:25.819>
actor = 'eric.smith'
assignee = 'none'
closed = True
closed_date = <Date 2009-08-20.16:12:25.830>
closer = 'eric.smith'
components = []
creation = <Date 2009-08-20.10:27:16.225>
creator = 'mvyskocil'
dependencies = []
files = []
hgrepos = []
issue_num = 6740
keywords = []
message_count = 2.0
messages = ['91766', '91779']
nosy_count = 2.0
nosy_names = ['eric.smith', 'mvyskocil']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue6740'
versions = ['Python 3.1']

[mvyskocil]

The compounded expressions with lambda functions are evaluated
incorrectly. The simple expressions, or a named functions are evaluated
good. The problem is only in the evaluation of compounded expressions.
It seems that after evaluate of the first lambda function the
evaluation of whole expression is stopped and not continue (see
cond_error, which may raises the exception during evaluation).

Python 3.1 (r31:73572, Aug 15 2009, 22:04:19)
[GCC 4.4.1 [gcc-4_4-branch revision 149935]] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> cond = (lambda x : x == 'foo') or (lambda x : x == 'bar')
>>> cond('foo')
True
>>> cond('bar')
False
>>> c1 = lambda x : x == 'foo'
>>> c1('foo')
True
>>> c2 = lambda x : x == 'bar'
>>> c2('bar')
True
>>> def ham(x): return x == 'foo'
...
>>> def spam(x): return x == 'bar'
...
>>> cond2 = lambda x : ham(x) or spam(x)
>>> cond2('foo')
True
>>> cond2('bar')
True
>>> cond2('ham')
False
>>> cond_error = (lambda x : x == 'foo') or (lambda x : y == 'bar')
>>> cond_error('d')
False

BTW: the same problem exists in Python 2.6.2
Python 2.6.2 (r262:71600, Aug 15 2009, 18:37:04)
[GCC 4.4.1 [gcc-4_4-branch revision 149935]] on linux2
Type "help", "copyright", "credits" or "license" for more information.

[ericvsmith]

This isn't the right forum to ask for help. You should try
comp.lang.python, where someone would be happy to explain this to you.

Having said that, here's the explanation:

This is not a bug.

Disregarding side effects, the expression:
a = b or c

is essentially the same as:
if b:
a = b
else:
a = c

So your code is:
if (lambda x : x == 'foo'):
cond = (lambda x : x == 'foo')
else:
cond = (lambda x : x == 'bar')

And since "(lambda x : x == 'foo')" evaluates to True (it's a lambda, so
it's not None), you're really saying:
cond = (lambda x : x == 'foo')

And your examples follow from that.

You can further verify this with your c1 and c2:
>>> cond = c1 or c2
>>> cond
<function <lambda> at 0x00B3EA30>
>>> cond is c1
True

So, you're just setting cond to the first of the 2 lambdas.