trouble with generators

[Hans-Peter Jansen]

Hi Pythonistas,

I'm stuck in a maze of new style classes and generators. While I love the
concepts, I obviously didn't grok them throughout.

I'm trying to generate a bunch of similar classes, where some are contained
in list attributes of others, e.g.:

class A:
def __init__(self):
self.id = 'A1'
self.b = [instances of B]

class B:
def __init__(self):
self.id = 'B1'

Here's the test code, I have:

#!/usr/bin/env python
# -*- coding: utf8 -*-

class A(object):
"A"
def __init__(self):
self.id = None
self.b = []

class B(object):
"B"
def __init__(self):
self.id = None

class Gen(object):
def records(self, cls):
for i in range(3):
setattr(cls, "id", "%s%s" % (cls.__doc__, i))
yield cls

def display(self, rec):
for i in rec.__dict__.keys():
if not i.startswith("_"):
print "%s: %s: %s" % (rec.__doc__, i, rec.__dict__)

class GenA(Gen):
def __init__(self):
self.genB = GenB()

def records(self):
for a in Gen.records(self, A()):
for b in self.genB.records():
#self.genB.display(b)
a.b.append(b)
#self.display(a)
yield a

class GenB(Gen):
def records(self):
return Gen.records(self, B())

# testing..

aRecs = []
bRecs = []

for i, r in enumerate(GenB().records()):
bRecs.append(r)
print i, r.id, r

for i, r in enumerate(GenA().records()):
aRecs.append(r)
print i, r.id, r
for b in r.b:
print b.id, b


Here's the commented output:
# even if I keep a reference to each rec, the object is reused:
0 B0 <__main__.B object at 0xb7bd0f8c>
1 B1 <__main__.B object at 0xb7bd0f8c>
2 B2 <__main__.B object at 0xb7bd0f8c>
# same here, with additional quadratic behavior, I do not understand
0 A0 <__main__.A object at 0xb7bd206c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
1 A1 <__main__.A object at 0xb7bd206c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd20ec>
2 A2 <__main__.A object at 0xb7bd206c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd0f8c>
B2 <__main__.B object at 0xb7bd0f8c>
B2 <__main__.B object at 0xb7bd0f8c>

I expected to get 3 different class objects from both sections, with each A
containing 3 different Bs in the latter section, but obviously got
something else.

Could some kind soul help me to distangle my mind twist here? Am I healable?

TIA, Pete

 

[Diez B. Roggisch]

Hi Pythonistas,

I'm stuck in a maze of new style classes and generators. While I love the
concepts, I obviously didn't grok them throughout.

I'm trying to generate a bunch of similar classes, where some are contained
in list attributes of others, e.g.:

All your code below shows that you don't create classes, but _instances_
of classes. So - is that what you mean to do? Or do you really want to
create classes?

class A:
def __init__(self):
self.id = 'A1'
self.b = [instances of B]

class B:
def __init__(self):
self.id = 'B1'

Here's the test code, I have:

#!/usr/bin/env python
# -*- coding: utf8 -*-

class A(object):
"A"
def __init__(self):
self.id = None
self.b = []

class B(object):
"B"
def __init__(self):
self.id = None

class Gen(object):
def records(self, cls):
for i in range(3):
setattr(cls, "id", "%s%s" % (cls.__doc__, i))
yield cls

def display(self, rec):
for i in rec.__dict__.keys():
if not i.startswith("_"):
print "%s: %s: %s" % (rec.__doc__, i, rec.__dict__)

class GenA(Gen):
def __init__(self):
self.genB = GenB()

def records(self):
for a in Gen.records(self, A()):

Here you pass an instance of A, not the class A.

for b in self.genB.records():
#self.genB.display(b)
a.b.append(b)
#self.display(a)
yield a

class GenB(Gen):
def records(self):
return Gen.records(self, B())

Same here - instance of B.

# testing..

aRecs = []
bRecs = []

for i, r in enumerate(GenB().records()):
bRecs.append(r)
print i, r.id, r

for i, r in enumerate(GenA().records()):
aRecs.append(r)
print i, r.id, r
for b in r.b:
print b.id, b


Here's the commented output:
# even if I keep a reference to each rec, the object is reused:
0 B0 <__main__.B object at 0xb7bd0f8c>
1 B1 <__main__.B object at 0xb7bd0f8c>
2 B2 <__main__.B object at 0xb7bd0f8c>

Sure - because you create one B-instance, and pass that to your generator.

That generator then simply sets the succinct id's on that object, and
returns it. But it is always the same instance!!

In a nutshell, you do this:

b = B()
res = []
for i in xrange(3):
b.id = i
res.append(b)

Always the same b.

What you seem to want is this
.... pass
....

>>> res = []
>>> for i in xrange(3):

.... class BSubClass(B):
.... pass
.... BSubClass.id = i
.... res.append(BSubClass)
....

>>> print [c.id for c in res] [0, 1, 2]
>>>

# same here, with additional quadratic behavior, I do not understand
0 A0 <__main__.A object at 0xb7bd206c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
1 A1 <__main__.A object at 0xb7bd206c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd20ec>
2 A2 <__main__.A object at 0xb7bd206c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd210c>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd20ec>
B2 <__main__.B object at 0xb7bd0f8c>
B2 <__main__.B object at 0xb7bd0f8c>
B2 <__main__.B object at 0xb7bd0f8c>

It's not quadratic - you add the same B to a.b in each generator run, so
the first run shows 3 times the B, then 6, then 9. And because its
always the same instance, printing them yields the same id for all - B2.

I expected to get 3 different class objects from both sections, with each A
containing 3 different Bs in the latter section, but obviously got
something else.

Could some kind soul help me to distangle my mind twist here? Am I healable?

I'm still not sure what you want - do you want instances created, or
classes? For the former, you need constructor calls on your classes, and
pass the class instead of an instance. Like this:


class B(object):
pass


def g(cls):
for i in xrange(3):
o = cls()
o.id = i
yield o

list(g(B))

Or you want really different classes (which is somewhat strange then to
create that hierarchy of yours with As containing Bs), which you can
accomplish using a class-statement in the generator as shown above.
There are other ways as well, but less intuitive I'd say.


Maybe stepping back and telling us what you want to accomplish here
would help

Diez

 

[Marc 'BlackJack' Rintsch]

class Gen(object):
def records(self, cls):
for i in range(3):
setattr(cls, "id", "%s%s" % (cls.__doc__, i))
yield cls

[…]

class GenA(Gen):
def __init__(self):
self.genB = GenB()

def records(self):
for a in Gen.records(self, A()):

Here you create an instance of `A` and pass that *instance* and not the
*class*. If you would pass the class here, you must create objects in
`Gen.records()`.

Ciao,
Marc 'BlackJack' Rintsch

 

[Hans-Peter Jansen]

Hi Diez,

first, thanks for your comprehensive answer.

All your code below shows that you don't create classes, but _instances_
of classes. So - is that what you mean to do?

Yes, exactly. Sorry for the confusion..

In a nutshell, you do this:

b = B()
res = []
for i in xrange(3):
b.id = i
res.append(b)

Always the same b.

What you seem to want is this
... pass
...

res = []
for i in xrange(3):

... class BSubClass(B):
... pass
... BSubClass.id = i
... res.append(BSubClass)
...

print [c.id for c in res]

[0, 1, 2]

I'm still not sure what you want - do you want instances created, or
classes? For the former, you need constructor calls on your classes, and
pass the class instead of an instance. Like this:


class B(object):
pass


def g(cls):
for i in xrange(3):
o = cls()
o.id = i
yield o

list(g(B))

Yes, that did the trick. Silly me. Rookie error. Here's what I was after:

#!/usr/bin/env python
# -*- coding: utf8 -*-

class A(object):
"A"
def __init__(self):
self.id = None
self.b = []

class B(object):
"B"
def __init__(self):
self.id = None

class Gen(object):
"Gen"
def records(self, cls):
for n in range(3):
i = cls()
i.id = "%s%s" % (i.__doc__, n)
yield i

class GenA(Gen):
def __init__(self):
self.genB = GenB()

def records(self):
for a in Gen.records(self, A):
for b in self.genB.records():
a.b.append(b)
yield a

class GenB(Gen):
def records(self):
return Gen.records(self, B)

aRecs = []
bRecs = []

for i, r in enumerate(GenB().records()):
bRecs.append(r)
print i, r.id, r

for i, r in enumerate(GenA().records()):
aRecs.append(r)
print i, r.id, r
for b in r.b:
print b.id, b

created pretty nice different _instances_ of what I wanted:
0 B0 <__main__.B object at 0xb7cacfec>
1 B1 <__main__.B object at 0xb7cae04c>
2 B2 <__main__.B object at 0xb7cae08c>

0 A0 <__main__.A object at 0xb7cae12c>
B0 <__main__.B object at 0xb7cae1ac>
B1 <__main__.B object at 0xb7cae1ec>
B2 <__main__.B object at 0xb7cae22c>
1 A1 <__main__.A object at 0xb7cae16c>
B0 <__main__.B object at 0xb7cae2ac>
B1 <__main__.B object at 0xb7cae2ec>
B2 <__main__.B object at 0xb7cae32c>
2 A2 <__main__.A object at 0xb7cae26c>
B0 <__main__.B object at 0xb7cae3ac>
B1 <__main__.B object at 0xb7cae3ec>
B2 <__main__.B object at 0xb7cae42c>

Didn't found the forest because all the trees. Thanks again.

Greetings to Berlin,
Pete

 

[Hans-Peter Jansen]

class Gen(object):
def records(self, cls):
for i in range(3):
setattr(cls, "id", "%s%s" % (cls.__doc__, i))
yield cls

[…]

class GenA(Gen):
def __init__(self):
self.genB = GenB()

def records(self):
for a in Gen.records(self, A()):

Here you create an instance of `A` and pass that *instance* and not the
*class*. If you would pass the class here, you must create objects in
`Gen.records()`.

Yes, that was my fault, as you both found.

Ciao,
Marc 'BlackJack' Rintsch

Thanks, Marc.

Cheers, Pete