Here is an example of the behavior:
------- code start -----------------------------------
#!/usr/bin/python
#bugtest - test of class attribute initiation
class Config:
a = 1
b = 2
c = 3
d = None
e = None
h = {'d' : 22, 'e' : 33}
def __init__(self, factor):
for attr in self.h.keys():
self.__dict__[attr] = self.h[attr] * factor
def moda(self):
self.a *= 5
c = Config(2)
print c.a, c.b, c.c, c.d, c.e
for attr in c.__dict__:
print 'c.%s = %s' % (attr, c.__dict__[attr])
print
c.moda()
print c.a, c.b, c.c, c.d, c.e
for attr in c.__dict__:
print 'c.%s = %s' % (attr, c.__dict__[attr])
print
------- code ends -----------------------------------
------- output starts -------------------------------
$ bugtest
1 2 3 44 66
c.e = 66
c.d = 44
5 2 3 44 66
c.a = 5
c.e = 66
c.d = 44
------- output ends ---------------------------------
What happened to c.a, c.b, and c.c when iterating thru
c.__dict__ ?
It appears that __dict__ members are not instantiated
until they are changed.
This precludes using __dict__ as the dictionary in
a formatted print statement. e.g.
print "c.a=%(a)s, c.b=%(b)s, c.c=%(c)s" % c.__dict__
Is this a bug or expected behavior? 4 1874
Ed Young wrote: What happened to c.a, c.b, and c.c when iterating thru c.__dict__ ?
It appears that __dict__ members are not instantiated until they are changed.
This precludes using __dict__ as the dictionary in a formatted print statement. e.g.
print "c.a=%(a)s, c.b=%(b)s, c.c=%(c)s" % c.__dict__
Is this a bug or expected behavior?
Expected behavior. What you're missing is the general way that Python
does attribute lookup. When c is an instance and you say c.x, Python
looks in c's __dict__ for an 'x' entry, then it looks in c's class's
__dict__ for an 'x' entry, then it looks (in a well-defined way) through
c's class's base classes, if any, for an 'x' entry in their __dict__
members.
When you defined
class C:
a = ...
b = ...
and so on, these are all _class_ attributes. When you instantiate a C
and then wrote self.a = ... in its methods, you instantiated _instance_
attributes on that instance. Class attributes are analogous to static
members/fields in other languages: class C: # class with two class attributes
.... a = 1
.... b = 2
.... c = C() d = C() c.a
1 d.a
1 c.a = 10 # change an instance attribute c.a
10 d.a
1 C.b = 20 # change a class attribute c.b
20 d.b
20 C.__dict__
{'a': 1, '__module__': '__main__', 'b': 20, '__doc__': None} c.__dict__
{'a': 10} d.__dict__
{}
--
Erik Max Francis && ma*@alcyone.com && http://www.alcyone.com/max/
__ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE
/ \ Nobody's on nobody's side
\__/ Florence, _Chess_
"Ed Young Here is an example of the behavior: ------- code start ----------------------------------- #!/usr/bin/python #bugtest - test of class attribute initiation
class Config: a = 1 b = 2 c = 3 d = None e = None h = {'d' : 22, 'e' : 33}
def __init__(self, factor): for attr in self.h.keys(): self.__dict__[attr] = self.h[attr] * factor
def moda(self): self.a *= 5
c = Config(2) print c.a, c.b, c.c, c.d, c.e for attr in c.__dict__: print 'c.%s = %s' % (attr, c.__dict__[attr]) print
c.moda() print c.a, c.b, c.c, c.d, c.e for attr in c.__dict__: print 'c.%s = %s' % (attr, c.__dict__[attr]) print ------- code ends ----------------------------------- ------- output starts ------------------------------- $ bugtest 1 2 3 44 66 c.e = 66 c.d = 44
5 2 3 44 66 c.a = 5 c.e = 66 c.d = 44 ------- output ends --------------------------------- What happened to c.a, c.b, and c.c when iterating thru c.__dict__ ?
They are up in C.__dict__
This precludes using __dict__ as the dictionary in a formatted print statement. e.g.
print "c.a=%(a)s, c.b=%(b)s, c.c=%(c)s" % c.__dict__
Not really. Use a wrapper to forward dict lookup requests
to getattr() which knows how/where to search for attributes:
class AttrDict:
def __init__(self, obj):
self.obj = obj
def __getitem__(sel f, key):
return getattr(self.ob j, key)
print "c.a=%(a)s, c.b=%(b)s, c.c=%(c)s" % AttrDict(c)
Is this a bug or expected behavior?
Expected.
Raymond Hettinger
Hi.
It's expected behaviour.
Let's go through your code with a few additional print statements to see if
we can demonstrate what's happening;
class Config:
a = 1
b = 2
c = 3
d = None
e = None
h = {'d' : 22, 'e' : 33}
def __init__(self, factor):
for attr in self.h.keys():
self.__dict__[attr] = self.h[attr] * factor
def moda(self):
self.a *= 5
c = Config(2)
# Here's what to pay attention to ...........
print "c.__dict__ : ", c.__dict__ #
this is the instance dictionary
print "c.__class__.__ dict__: ", c.__class__.__d ict__ # this is the class
dictionary
print c.a, c.b, c.c, c.d, c.e
for attr in c.__dict__:
print 'c.%s = %s' % (attr, c.__dict__[attr])
print
c.moda()
print "c.moda() --------------"
print "c.__dict__ : ", c.__dict__
print "c.__class__.__ dict__: ", c.__class__.__d ict__
print c.a, c.b, c.c, c.d, c.e
for attr in c.__dict__:
print 'c.%s = %s' % (attr, c.__dict__[attr])
print
Now, here's the output with #annotations:
c.__dict__: {'e': 66, 'd': 44}
c.__class__.__d ict__: {'a': 1, 'moda': <function moda at 0x015209B0>,
'__module__': '__main__', 'b': 2, 'e': None, 'd': None, 'h': {'e': 33, 'd':
22}, 'c': 3, '__init__': <function __init__ at 0x01520B30>, '__doc__': None}
1 2 3 44 66
c.e = 66
c.d = 44
# Okay. We can see that the values for 'a', 'b', 'c' were all found
# in the class dictionary of instance c, while 'd', and 'e' were
# found in the instance dictionary of c. More on this later....
# Now we're about to call moda() ....
c.moda() --------------
# What's changed?
c.__dict__: {'a': 5, 'e': 66, 'd': 44}
c.__class__.__d ict__: {'a': 1, 'moda': <function moda at 0x015209B0>,
'__module__': '__main__', 'b': 2, 'e': None, 'd': None, 'h': {'e': 33, 'd':
22}, 'c': 3, '__init__': <function __init__ at 0x01520B30>, '__doc__': None}
5 2 3 44 66
c.a = 5
c.e = 66
c.d = 44
# This time only 'b' and 'c''s values were pulled from instance c's class'
dictionary.
# What about 'a'? 'a' was pulled from c's instance dictionary. Nothing's
changed
# for 'd' and 'e'.
Okay then. What's going on?
class Config:
a = 1
b = 2
c = 3
d = None
e = None
h = {'d' : 22, 'e' : 33}
The code above adds class variables a - h to the class Config. So, if you
have an instance c of Config,
variables a-h are stored in c's class dictionary (c.__class__.__ dict__) and
NOT c's instance dictionary
(c.__dict__). Moving on...
def __init__(self, factor):
for attr in self.h.keys():
self.__dict__[attr] = self.h[attr] * factor
Inside the constructor, you call self.h.keys(). To find self.h, Python looks
first in self.__dict__. But 'h' isn't there.
Next it looks in self.__class__. __dict__. That's were 'h' is! Now this:
self.__dict__[attr] = self.h[attr] * factor
Here, you're assigning NEW attributes 'd' and 'e' to self's __dict__. What
you are not doing is assigning new values to class variables 'd' and 'e' in
self.__class__. __dict__ .
def moda(self):
self.a *= 5
Something similar is happening in here. This one is a bit more complicated.
self.a *= 5
is the same as
self.a = self.a * 5
What does this really mean? Well,
self.a = ....
is equivalent to
self.__dict__['a'] = ....
But
self.a = self.a ....
is not necessarily equivalent to
self.__dict__['a'] = self.__dict__['a']
because the self.a on the right hand side of the assignment has to be looked
up by Python. And, as we showed earlier,
look up starts with self.__dict__. But 'a' is not yet a key in that
dictionary, so we move up to self.__class__. __dict__.
That's where 'a' is! It's value is '1', so we get
self.__dict__['a'] = 1*5
^
self.__class__. __dict__['a']
We finish the evaluation, and assign 5 to self.__dict__['a'], creating a new
instance variable.
The class variable 'a' is unchanged. If you call c.moda() again later then,
that time, Python's lookup
would find 'a' in self.__dict__, and the expression self.a *= 5 would be
equivalent to
self.__dict__['a'] = 5*5
^
self.__dict__['a']
So, the thing is, yes the behaviour is expected, if you know what behaviour
to expect ...
Okay, then. Hopefully that was helpful.
Sean
Thank you all for the kind and detailed explanations.
I now have a thorough understanding of the mechanism
behind attribute lookup. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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