Subclass str: where is the problem?

[pascal.parent]

Hello, can anybody explain/help me:

Python 2.4.2 (#2, Sep 30 2005, 21:19:01)
[GCC 4.0.2 20050808 (prerelease) (Ubuntu 4.0.1-4ubuntu8)] on linux2


class Upper(str):
def __new__(cls, value):
return str.__new__(cls, value.upper())

u = Upper('test')
u
'TEST'
type(u)
<class '__main__.Upper'>
u = Upper('')
u
''
type(u)
<class '__main__.Upper'>


All seems to be ok...

class MyObject(object):
def __init__(self, dictionary = {}):
self.id = dictionary.get('id', '')
def __setattr__(self, attribute, value):
value = type(value) is type('') and Upper(value) or value
object.__setattr__(self, attribute, value)

m = MyObject({'id': 'test'})
m.id
'TEST'
type(m.id)
<class '__main__.Upper'>
m = MyObject()
m.id
''
type(m.id)
<type 'str'>

Why is m.id a str ?
Thanks.

 

[harold]

Hello, can anybody explain/help me:

Python 2.4.2 (#2, Sep 30 2005, 21:19:01)
[GCC 4.0.2 20050808 (prerelease) (Ubuntu 4.0.1-4ubuntu8)] on linux2


class Upper(str):
def __new__(cls, value):
return str.__new__(cls, value.upper())

u = Upper('test')
u
'TEST'
type(u)
<class '__main__.Upper'>
u = Upper('')
u
''
type(u)
<class '__main__.Upper'>


All seems to be ok...

class MyObject(object):
def __init__(self, dictionary = {}):
self.id = dictionary.get('id', '')
def __setattr__(self, attribute, value):
value = type(value) is type('') and Upper(value) or value
object.__setattr__(self, attribute, value)

m = MyObject({'id': 'test'})
m.id
'TEST'
type(m.id)
<class '__main__.Upper'>
m = MyObject()
m.id
''
type(m.id)
<type 'str'>

Why is m.id a str ?

Because Upper(value) will be False in the line

value = type(value) is type('') and Upper(value) or value

and thus, you assign value itself to value again.
rewrite it for exmaple in this way:

def __setattr__(self, attribute, value):
if type(value) is type('') :
value = Upper(value)
object.__setattr__(self, attribute, value)

 

[pascal.parent]

This is good... try this:

value = 'test'
value
'test'
type(value)
<type 'str'>
value = type(value) is type('') and Upper(value) or value
value
'TEST'
type(value)
<class '__main__.Upper'>

value = 1
value
1
type(value)
<type 'int'>
value = type(value) is type('') and Upper(value) or value
value
1
type(value)
<type 'int'>

 

[Peter Otten]

This is good... try this:

value = 'test'
value
'test'
type(value)
<type 'str'>
value = type(value) is type('') and Upper(value) or value
value
'TEST'
type(value)
<class '__main__.Upper'>

Try again with

value = ""

This makes Upper(value) a False value in a boolean context:
.... pass
....<type 'str'>

versus
<class '__main__.Upper'>

Peter

 

[pascal.parent]

Effectively.
Thanks a lot Peter and Harold.

 

[harold]

Hi Pascal,

Indeed, the example you show work corrctly. But in the
code you posted before, you wonder about the behavior
of these lines:

m = MyObject()
m.id
''
type(m.id)
<type 'str'>

So what happens when you provide the empty string '' to your and-or
construct?
value = ''
value = type(value) is type('') and Upper(value) or value

As you can easily check, bool('') resolves to False. Thus, your line
becomes:
value = False and False or value
Python first evaluates the and expression, which resolves to False. The
or
expression evaluates the second argument and returns that one as the
result
if and only if the first one evaluates to False, which is the case
here. So the
result of False or value is value. You can check thatTrue
so, in you code, the empty string gets indeed assign as value again.

Be carefull with the condition/and/or chain! You must be 110% sure,
that
the desired return value in case of condition==True can never evaluate
to False!

- harold -