Reflection in python

Question

I'm trying to find some info on reflection in python. I found a wikipedia article which gave this as a code snippet:

# without reflection
Foo().hello()

# with reflection
getattr(globals()['Foo'](), 'hello')()

I wasn't able to get this to work. What I really need is a way to just instantiate the object. So if I have a string 'Foo' I want to be able to get an object of type Foo. Like in java I could say something like: Class.forName("Foo")

Just found this...wonder why I couldn't find this before: Does python have an equivalent to Java Class.forName()?

you are almost there: fooobj = globals()['Foo']() ... fooobj.hello() — ralf.w.
– ralf.w., Commented Jan 5, 2011 at 7:13

Karl Knechtel · Accepted Answer · 2011-01-05 07:03:26Z

12

What I really need is a way to just instantiate the object.

That's what the globals()['Foo']() part does.

And it works for me:

>>> class Foo:
...   def __init__(self): print "Created a Foo!"
...
>>> globals()['Foo']()
Created a Foo!
<__main__.Foo instance at 0x02A33350>
>>>

answered Jan 5, 2011 at 7:03

Karl Knechtel

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5 Comments

JPC Over a year ago

What is the difference between f = globals()['Foo']() and f = ['Foo']()? What does globals mean

Senthil Kumaran Over a year ago

@JPC, second is a syntax error. golbals() refer to the namespace of your module (which is a dictionary) and namespace is how Python deal with it's programs/modules. By golbals()['Foo'] you are accessing the key called 'Foo' in the glabals() dictionary.

JPC Over a year ago

That makes sense, thanks. Reflection in python is so much easier than in java!

Karl Knechtel Over a year ago

globals is a function that returns a dict of the global variables. globals() calls that function, so now you have that dict. globals()['Foo'] looks up the 'Foo' key in that dict, which gives you the value of the Foo global variable.

Karl Knechtel Over a year ago

Coming back to this years later in response to a random upvote: ['Foo']() isn't actually a syntax error, but it will cause a TypeError at runtime. It means "make a list with one element in it, the string 'Foo', call that list, and let f be a name for the result of the call". It fails because lists are not callable.

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