Re: Instantiating an object when the type is only known at runtime

[Carsten Haese]

On Tue, 2006-10-03 at 18:19, Samuel wrote:
 Thanks, that's what I was looking for.
 
   m = __import__( StringIO )
   x = getattr( m, StringIO )()
   x
  StringIO.StringIO instance at 0x00A47710
  
 
 For the records: If the module is already loaded, this also works:
 
 if my_type_is_not_yet_loaded:
 module = __import__(type)
 obj= getattr(module, type)
 else:
 obj= globals().get(type)
 resource = obj(my_arg1, my_arg2)

You seem to be under the impression that importing an already imported
module is a hideously expensive operation that must be avoided at all
costs. It's not. If you import an already imported module, python simply
returns the module object from sys.modules without executing any of the
module's code.

Your code will be much easier to read if you eliminate the
my_type_is_not_yet_loaded check, whatever that may be under the hood,
and always perform the __import__. It's also likely to be at least as
fast, but I couldn't run a timing comparison even if I wanted to because
you didn't post working code.

-Carsten


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Instantiating an object when the type is only known at runtime

[Samuel]

Hi,

I am trying to replace the eval() in the following code:

def myfunc(type, table):
module   = __import__(type)
type = 'module' + '.' + type
obj  = eval(type)
return obj(row[table.c.name], row[table.c.handle])

I am out of ideas. Any hints?

Thanks,
-Samuel

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Re: Instantiating an object when the type is only known at runtime

[Rob Williscroft]

Samuel wrote in news:[EMAIL PROTECTED] 
in comp.lang.python:

 Hi,
 
 I am trying to replace the eval() in the following code:
 
 def myfunc(type, table):
 module   = __import__(type)
 type = 'module' + '.' + type
 obj  = eval(type)
 return obj(row[table.c.name], row[table.c.handle])
 

 m = __import__( StringIO )
 x = getattr( m, StringIO )()
 x
StringIO.StringIO instance at 0x00A47710
 

Rob.
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Re: Instantiating an object when the type is only known at runtime

[Terry Reedy]

Samuel [EMAIL PROTECTED] wrote in message 
news:[EMAIL PROTECTED]
 Hi,

 I am trying to replace the eval() in the following code:

 def myfunc(type, table):
module   = __import__(type)
type = 'module' + '.' + type
obj  = eval(type)
return obj(row[table.c.name], row[table.c.handle])

 I am out of ideas. Any hints?

Perhaps what you need is a dict 'types' mapping strings to types/classes.
Then the last two lines might become

  return types[type](row[table.c.name], row[table.c.handle])

The trick of mapping names to callables for runtime choice of what to call 
has several uses.

Terry Jan Reedy



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Re: Instantiating an object when the type is only known at runtime

[Samuel]

Thanks, that's what I was looking for.

  m = __import__( StringIO )
  x = getattr( m, StringIO )()
  x
 StringIO.StringIO instance at 0x00A47710
 

For the records: If the module is already loaded, this also works:

if my_type_is_not_yet_loaded:
module = __import__(type)
obj= getattr(module, type)
else:
obj= globals().get(type)
resource = obj(my_arg1, my_arg2)

Thanks again,
-Samuel

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