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Python for experienced programmers |
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2.5. Getting object references with getattr
You already know that Python functions are objects. What you don't know is that you can get a reference to a function without knowing its name until run-time, using the getattr function.
Example 2.12. Introducing getattr
>>> li = ["Larry", "Curly"]
>>> li.pop
<built-in method pop of list object at 010DF884>
>>> getattr(li, "pop")
<built-in method pop of list object at 010DF884>
>>> getattr(li, "append")("Moe")
>>> li
["Larry", "Curly", "Moe"]
>>> getattr({}, "clear")
<built-in method clear of dictionary object at 00F113D4>
>>> getattr((), "pop")
Traceback (innermost last):
File "<interactive input>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'pop'
| This gets a reference to the pop method of the list. Note that this is not calling the pop method; that would be li.pop(). This is the method itself. |
| This also returns a reference to the pop method, but this time, the method name is specified as a string argument to the getattr function. getattr is an incredibly useful built-in function which returns any attribute of any object. In this case, the object is a list, and the attribute is the pop method. |
| In case it hasn't sunk in just how incredibly useful this is, try this: the return value of getattr is the method, which you can then call just as if you had said li.append("Moe") directly. But you didn't call the function directly; you specified the function name as a string instead. |
| getattr also works on dictionaries. |
| In theory, getattr would work on tuples, except that tuples have no methods, so getattr will raise an exception no matter what attribute name you give. |
getattr isn't just for built-in datatypes. It also works on modules.
Example 2.13. getattr in apihelper.py
>>> import odbchelper
>>> odbchelper.buildConnectionString
<function buildConnectionString at 00D18DD4>
>>> getattr(odbchelper, "buildConnectionString")
<function buildConnectionString at 00D18DD4>
>>> object = odbchelper
>>> method = "buildConnectionString"
>>> getattr(object, method)
<function buildConnectionString at 00D18DD4>
>>> type(getattr(object, method))
<type 'function'>
>>> import types
>>> type(getattr(object, method)) == types.FunctionType
1
| This returns a reference to the buildConnectionString function in the odbchelper module, which we studied in Chapter 1. (The hex address you see is specific to my machine; your output will be different.) |
| Using getattr, we can get the same reference to the same function. In general, getattr(object, "attribute") is equivalent to object.attribute. If object is a module, then attribute can be anything defined in the module: a function, class, or global variable. |
| And this is what we actually use in the help function. object is passed into the function as an argument; method is a string which is the name of a method or function. |
| In this case, method is the name of a function, which we can prove by getting its type. |