Dive Into Python

4.4. Getting Object References With getattr

4.4. 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, by using the getattr function.

Example 4.10. 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 that 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.

4.4.1. `getattr` with Modules

getattr isn't just for built-in datatypes. It also works on modules.

Example 4.11. The `getattr` Function 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
True
>>> callable(getattr(object, method))            
True

	This returns a reference to the `buildConnectionString` function in the `odbchelper` module, which you studied in Chapter 2, Your First Python Program. (The hex address you see is specific to my machine; your output will be different.)
	Using `getattr`, you 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 you actually use in the `info` 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 you can prove by getting its `type`.
	Since `method` is a function, it is callable.

4.4.2. `getattr` As a Dispatcher

A common usage pattern of getattr is as a dispatcher. For example, if you had a program that could output data in a variety of different formats, you could define separate functions for each output format and use a single dispatch function to call the right one.

For example, let's imagine a program that prints site statistics in HTML, XML, and plain text formats. The choice of output format could be specified on the command line, or stored in a configuration file. A statsout module defines three functions, output_html, output_xml, and output_text. Then the main program defines a single output function, like this:

Example 4.12. Creating a Dispatcher with `getattr`


import statsout

def output(data, format="text"):                              
    output_function = getattr(statsout, "output_%s" % format) 
    return output_function(data)

	The `output` function takes one required argument, `data`, and one optional argument, `format`. If `format` is not specified, it defaults to `text`, and you will end up calling the plain text output function.
	You concatenate the `format` argument with "output_" to produce a function name, and then go get that function from the `statsout` module. This allows you to easily extend the program later to support other output formats, without changing this dispatch function. Just add another function to `statsout` named, for instance, `output_pdf`, and pass "pdf" as the `format` into the `output` function.
	Now you can simply call the output function in the same way as any other function. The `output_function` variable is a reference to the appropriate function from the `statsout` module.

Did you see the bug in the previous example? This is a very loose coupling of strings and functions, and there is no error checking. What happens if the user passes in a format that doesn't have a corresponding function defined in statsout? Well, getattr will return None, which will be assigned to output_function instead of a valid function, and the next line that attempts to call that function will crash and raise an exception. That's bad.

Luckily, getattr takes an optional third argument, a default value.

Example 4.13. `getattr` Default Values


import statsout

def output(data, format="text"):
    output_function = getattr(statsout, "output_%s" % format, statsout.output_text)
    return output_function(data)

This function call is guaranteed to work, because you added a third argument to the call to getattr. The third argument is a default value that is returned if the attribute or method specified by the second argument wasn't found.

As you can see, getattr is quite powerful. It is the heart of introspection, and you'll see even more powerful examples of it in later chapters.

Dive Into Python

4.4. Getting Object References With getattr

Example 4.10. Introducing getattr

4.4.1. getattr with Modules

Example 4.11. The getattr Function in apihelper.py

4.4.2. getattr As a Dispatcher

Example 4.12. Creating a Dispatcher with getattr

Example 4.13. getattr Default Values