FAQ
I am a C++ developer with only a little experience using Python. I
want to create a Python class where by I can construct an instance from
that class based on one of two different object types.

For example, if I were programming in C++, I would do the something
like the following:

class MyClass
{
public:
MyClass(const SomeType& type);
MyClass(const SomeOtherType& type);
...
};

In Python I cannot have two constructors that each take a single
argument as Python does not distinguish the type of objects that are
passed to functions.

One thought I had was to use the isinstance method such as this:

class MyClass:
__init__(self, object):
if isinstance(object, SomeType):
#Initialize based on SomeType object
...

elif isinstance(object, SomeOtherType):
#Initialize base on SomeOtherType object
...

else:
#Raise some kind of exception
...

Some research I've done on the Internet indicates that the use of the
isinstance method can be problematic, and I'm not sure if it is the
best approach to solving my problem.

What is the best way for me to implement this type of functionality in
Python?

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  • Steven Bethard at May 8, 2005 at 10:21 pm

    tron.thomas at verizon.net wrote:
    I am a C++ developer with only a little experience using Python. I
    want to create a Python class where by I can construct an instance from
    that class based on one of two different object types.

    For example, if I were programming in C++, I would do the something
    like the following:

    class MyClass
    {
    public:
    MyClass(const SomeType& type);
    MyClass(const SomeOtherType& type);
    ...
    };
    How about using a classmethod as an alternate constructor:

    py> class C(object):
    ... def __init__(self, i):
    ... self.i = i
    ... @classmethod
    ... def fromstr(cls, s):
    ... return cls(int(s))
    ...
    py> C(1).i
    1
    py> C.fromstr('2').i
    2

    STeVe
  • James Stroud at May 8, 2005 at 10:28 pm

    On Sunday 08 May 2005 03:05 pm, tron.thomas at verizon.net wrote:
    I am a C++ developer with only a little experience using Python. I
    want to create a Python class where by I can construct an instance from
    that class based on one of two different object types.

    For example, if I were programming in C++, I would do the something
    like the following:

    class MyClass
    {
    public:
    MyClass(const SomeType& type);
    MyClass(const SomeOtherType& type);
    ...
    };

    In Python I cannot have two constructors that each take a single
    argument as Python does not distinguish the type of objects that are
    passed to functions.

    One thought I had was to use the isinstance method such as this:

    class MyClass:
    __init__(self, object):
    if isinstance(object, SomeType):
    #Initialize based on SomeType object
    ...

    elif isinstance(object, SomeOtherType):
    #Initialize base on SomeOtherType object
    ...

    else:
    #Raise some kind of exception
    ...
    Some research I've done on the Internet indicates that the use of the
    isinstance method can be problematic, and I'm not sure if it is the
    best approach to solving my problem.

    What is the best way for me to implement this type of functionality in
    Python?
    In case you haven't found it: <http://www.canonical.org/~kragen/isinstance/>

    Can both of these classes (be modified to/subclassed to) support the same
    interface such that MyClass.__init__ will not care which is passed? I believe
    this would be the best solution (read: "my favorite solution"). If you know
    what type of object "object" is (BTW, a keyword in 2.3 and later, I believe),
    then one approach is to initialize with a blank MyClass instance and use
    "fill_with_SomeType()" and "fill_with_SomeOtherType()" methods.

    I think the least elegant approach is to test for interface compatibility,
    e.g.:

    try:
    self.avalue = isinstance.get_avalue()
    except NameError:
    self.avalue = isinstance.get_anothervalue()

    But this may get out of hand with many different possibilites.

    James

    --
    James Stroud
    UCLA-DOE Institute for Genomics and Proteomics
    Box 951570
    Los Angeles, CA 90095

    http://www.jamesstroud.com/
  • James Stroud at May 8, 2005 at 10:38 pm

    On Sunday 08 May 2005 03:28 pm, James Stroud wrote:
    ? ?try:
    ? ? ?self.avalue = isinstance.get_avalue()
    ? ?except NameError:
    ? ? ?self.avalue = isinstance.get_anothervalue()
    I have no idea where I copied those "isinstance"s from. Also, the except
    should be an AttributeError. Here is a retry:

    ? ?try:
    ? ? ?self.avalue = aninstance.get_avalue()
    ? ?except AttributeError:
    ? ? ?self.avalue = aninstance.get_anothervalue()

    --
    James Stroud
    UCLA-DOE Institute for Genomics and Proteomics
    Box 951570
    Los Angeles, CA 90095

    http://www.jamesstroud.com/
  • J C Lawrence at May 8, 2005 at 10:59 pm

    tron.thomas at verizon.net wrote:

    I am a C++ developer with only a little experience using Python. I
    want to create a Python class where by I can construct an instance from
    that class based on one of two different object types.
    The approaches I've seen used are to use a new class method as an
    alternate ctor with a special name, and to use the types module for type
    comparison within such a ctor.

    --
    J C Lawrence They said, "You have a blue guitar,
    ---------(*) You do not play things as they are."
    claw at kanga.nu The man replied, "Things as they are
    http://www.kanga.nu/~claw/ Are changed upon the blue guitar."
  • Steven Bethard at May 9, 2005 at 5:19 am

    James Stroud wrote:
    If you know what type of object "object" is
    (BTW, a keyword in 2.3 and later, I believe)
    Not a keyword, but a builtin as of 2.2.

    STeVe

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postedMay 8, '05 at 10:05p
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