something wrong with isinstance

M

maksym.kaban

Hi there.
now i'm a complete newbie for python, and maybe my problem is stupid
but i cannot solve it myself

i have an object of class GeoMap which contains lists with objects of
GeoMapCell (i will not explain what they should do, hope its not
important). Then i want to serialize these objects to json notation.
So i imported json module and as shown in docs for it extended
JSONEncoder class. Look at the code below

##main module
from worldmap import GeoMap, GeoMapCell

import testdata
import json

class GeoMapEncoder(json.JSONEncoder):
def default(self, obj):
if isinstance(obj, GeoMap):
return None
return json.JSONEncoder.default(self, obj)

def main():
print(json.dumps(2 + 5j, cls=ComplexEncoder))

geomap = testdata.createTestMap()
print(json.dumps(geomap, cls=GeoMapEncoder))
pass

if __name__ == '__main__':
main()

===========

##worldmap module
class GeoMap:
cells = []
activerow = 0
activecol = 0

def addCell(self, acell):
if len(self.cells) == 0:
self.cells.append([])
self.activerow = 0
acell.col = self.activerow
acell.row = self.activecol
self.cells[self.activerow].append(acell)
self.activecol += 1

def addRow(self):
self.cells.append([])
self.activerow += 1;
self.activecol = 0;

class GeoMapCell:
neighbours = (None, None, None, None, None, None, )
col = 0
row = 0

The problem is in here.

class GeoMapEncoder(json.JSONEncoder):
def default(self, obj):
if isinstance(obj, GeoMap): ## <======= isinstance doesnot
work as i expected
return None
return json.JSONEncoder.default(self, obj)

Though obj is object of GeoMap class, isinstance returns False. Where
was i mistaken. If i shouldn't use isinstance, then what function
would check class of object?

Oh, maybe its important. I'm working on WinXP SP3, Python 3.0, IDE -
PyScript
 
P

Paul McGuire

Hi there.
now i'm a complete newbie for python, and maybe my problem is stupid
but i cannot solve it myself

i have an object of class GeoMap which contains lists with objects of
GeoMapCell (i will not explain what they should do, hope its not
important). Then i want to serialize these objects to json notation.
So i imported json module and as shown in docs for it extended
JSONEncoder class.  Look at the code below

##main module
from worldmap import GeoMap, GeoMapCell

import testdata
import json

class GeoMapEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, GeoMap):
            return None
        return json.JSONEncoder.default(self, obj)

def main():
    print(json.dumps(2 + 5j, cls=ComplexEncoder))

    geomap = testdata.createTestMap()
    print(json.dumps(geomap, cls=GeoMapEncoder))
    pass

if __name__ == '__main__':
    main()

===========

##worldmap module
class GeoMap:
    cells = []
    activerow = 0
    activecol = 0

    def addCell(self, acell):
        if len(self.cells) == 0:
          self.cells.append([])
          self.activerow = 0
        acell.col = self.activerow
        acell.row = self.activecol
        self.cells[self.activerow].append(acell)
        self.activecol += 1

    def addRow(self):
        self.cells.append([])
        self.activerow += 1;
        self.activecol = 0;

class GeoMapCell:
    neighbours = (None, None, None, None, None, None, )
    col = 0
    row = 0

The problem is in here.

class GeoMapEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, GeoMap):  ## <=======   isinstance doesnot
work as i expected
            return None
        return json.JSONEncoder.default(self, obj)

 Though obj is object of GeoMap class, isinstance returns False. Where
was i mistaken. If i shouldn't use isinstance, then what function
would check class of object?

Oh, maybe its important. I'm working on WinXP SP3, Python 3.0, IDE -
PyScript
Click to expand...

Here's a crazy idea out of left field. Just before calling
isinstance, why not try:

print(type(obj))
print(str(obj))

This may illuminate the unexpected behavior, you'll find out just what
obj has in it.

-- Paul
 
M

maksym.kaban

Here's a crazy idea out of left field.  Just before calling
isinstance, why not try:

print(type(obj))
print(str(obj))

This may illuminate the unexpected behavior, you'll find out just what
obj has in it.

-- Paul
Click to expand...

Well the output of
print(type(obj))
print(str(obj))
Click to expand...

was

<class 'worldmap.GeoMap'>
<worldmap.GeoMap object at 0x01DD17B0>

Just looks well, isn't it? i have no idea what's wrong
 
R

redbaron

Don't really sure, but try to define your class as new-style one.
Like
class GeoMap(object):
...
 
P

Paul McGuire

Well the output of


was

<class 'worldmap.GeoMap'>
<worldmap.GeoMap object at 0x01DD17B0>

Just looks well, isn't it? i have no idea what's wrong
Click to expand...

So then how do you know isinstance is evaluating to False? And why do
you return None if it evals to True? Can you drop a print command
inside the if block just before returning None, and another print just
before returning the default default?

I also would recommend using a debugger like winpdb when in a spot
like this - despite the name, it is not Windows-only, and you can step
through this code and evaluate variables and expressions inline.

-- Paul
 
D

Diez B. Roggisch

Hi there.
now i'm a complete newbie for python, and maybe my problem is stupid
but i cannot solve it myself

i have an object of class GeoMap which contains lists with objects of
GeoMapCell (i will not explain what they should do, hope its not
important). Then i want to serialize these objects to json notation.
So i imported json module and as shown in docs for it extended
JSONEncoder class. Look at the code below

##main module
from worldmap import GeoMap, GeoMapCell

import testdata
import json

class GeoMapEncoder(json.JSONEncoder):
def default(self, obj):
if isinstance(obj, GeoMap):
return None
return json.JSONEncoder.default(self, obj)

def main():
print(json.dumps(2 + 5j, cls=ComplexEncoder))

geomap = testdata.createTestMap()
print(json.dumps(geomap, cls=GeoMapEncoder))
pass

if __name__ == '__main__':
main()

===========

##worldmap module
class GeoMap:
cells = []
activerow = 0
activecol = 0

def addCell(self, acell):
if len(self.cells) == 0:
self.cells.append([])
self.activerow = 0
acell.col = self.activerow
acell.row = self.activecol
self.cells[self.activerow].append(acell)
self.activecol += 1

def addRow(self):
self.cells.append([])
self.activerow += 1;
self.activecol = 0;

class GeoMapCell:
neighbours = (None, None, None, None, None, None, )
col = 0
row = 0

The problem is in here.

class GeoMapEncoder(json.JSONEncoder):
def default(self, obj):
if isinstance(obj, GeoMap): ## <======= isinstance doesnot
work as i expected
return None
return json.JSONEncoder.default(self, obj)

Though obj is object of GeoMap class, isinstance returns False. Where
was i mistaken. If i shouldn't use isinstance, then what function
would check class of object?

Oh, maybe its important. I'm working on WinXP SP3, Python 3.0, IDE -
PyScript
Click to expand...


I think you have a common problem here that occurs when using a script
with more that non-trivial contents as main-script.


What happens is this: you have a

__main__.GeoMap, which is the one tested against in isinstance.

*And* you have a <mainmodule>.GeoMap (replace <mainmodule> with the
actual name) that is imported & used from the other module.


A simple testscript illustrates the issue:


### test.py ####

class Foo(object):
pass


if __name__ == "__main__":
import test
print Foo == test.Foo


Run it from inside the directory you saved it in.

To work around this issue, simply create a bootstrap-main-module that
has not much in it.

Diez
 
B

Bruno Desthuilliers

(e-mail address removed) a écrit :
Hi there.
now i'm a complete newbie for python, and maybe my problem is stupid
but i cannot solve it myself
Click to expand...

Others already addressed your problem (cf Paul and Diez answers). I'll
just allow myself to point a couple other potential problems with your code:
##worldmap module
class GeoMap:
cells = []
activerow = 0
activecol = 0
Click to expand...


Attributes defined at the class level are *class* attributes, shared by
all instances of the class - that is, all instances will access the one
same 'cells' list. Instance attributes are canonically created in the
initialize method (__init__) that is automagically called on
instanciation. IOW, you want to replace the above lines with:

def __init__(self):
self.cells = []
self.activerow = 0
self.activecol = 0

def addCell(self, acell):
if len(self.cells) == 0:
Click to expand...

An empty list evals to False in a boolean context, so the above can be
simply expressed as:
if not self.cells:
self.cells.append([])
self.activerow = 0
acell.col = self.activerow
acell.row = self.activecol
self.cells[self.activerow].append(acell)
self.activecol += 1

def addRow(self):
self.cells.append([])
self.activerow += 1;
self.activecol = 0;

class GeoMapCell:
neighbours = (None, None, None, None, None, None, )
col = 0
row = 0
Click to expand...

Same remark as above. You want to move all this code to the __init__ method.
 
C

Carl Banks

Don't really sure, but try to define your class as new-style one.
Like
class GeoMap(object):
   ...
Click to expand...

Well, the OP said he was using Python 3.0, where all classes are new-
style classes.

But that brings up another very slight possibility, though not a very
likely one in this case: the behavior of isinstance can be
customized. It can happen unbeknownst to a user who subclasses a
class that does that.


Carl Banks
 
G

Gabriel Genellina

Well, the OP said he was using Python 3.0, where all classes are new-
style classes.

But that brings up another very slight possibility, though not a very
likely one in this case: the behavior of isinstance can be
customized. It can happen unbeknownst to a user who subclasses a
class that does that.
Click to expand...

Really? I didn't know that -- how do you do that?
 

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