Using Keyword-Only Arguments

It’s interesting to notice that this example could be coded with Python 3.0 keyword-only arguments, described earlier in this chapter, to automatically validate configuration arguments:

# Use keyword-only args

def print30(*args, sep=' ', end='\n', file=sys.stdout):
    output = ''
    first  = True
    for arg in args:
        output += ('' if first else sep) + str(arg)
        first = False
    file.write(output + end)

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This version works the same as the original, and it’s a prime example of how keyword-only arguments come in handy. The original version assumes that all positional arguments are to be printed, and all keywords are for options only. That’s almost sufficient, but any extra keyword arguments are silently ignored. A call like the following, for instance, will generate an exception with the keyword-only form:

>>> print30(99, name='bob')
TypeError: print30() got an unexpected keyword argument 'name'

but will silently ignore the name argument in the original version. To detect superfluous keywords manually, we could use dict.pop() to delete fetched entries, and check if the dictionary is not empty. Here is an equivalent to the keyword-only version:

# Use keyword args deletion with defaults

def print30(*args, **kargs):
    sep  = kargs.pop('sep', ' ')
    end  = kargs.pop('end', '\n')
    file = kargs.pop('file', sys.stdout)
    if kargs: raise TypeError('extra keywords: %s' % kargs)
    output = ''
    first  = True
    for arg in args:
        output += ('' if first else sep) + str(arg)
        first = False
    file.write(output + end)

This works as before, but it now catches extraneous keyword arguments, too:

>>> print30(99, name='bob')
TypeError: extra keywords: {'name': 'bob'}

This version of the function runs under Python 2.6, but it requires four more lines of code than the keyword-only version. Unfortunately, the extra code is required in this case—the keyword-only version only works on 3.0, which negates most of the reason that I wrote this example in the first place (a 3.0 emulator that only works on 3.0 isn’t incredibly useful!). In programs written to run on 3.0, though, keyword-only arguments can simplify a specific category of functions that accept both arguments and options. For another example of 3.0 keyword-only arguments, be sure to see the upcoming iteration timing case study in Chapter 20.

Why You Will Care: Keyword Arguments

As you can probably tell, advanced argument-matching modes can be complex. They are also entirely optional; you can get by with just simple positional matching, and it’s probably a good idea to do so when you’re starting out. However, because some Python tools make use of them, some general knowledge of these modes is important.

For example, keyword arguments play an important role in tkinter, the de facto standard GUI API for Python (this module’s name is Tkinter in Python 2.6). We touch on tkinter only briefly at various points in this book, but in terms of its call patterns, keyword arguments set configuration options when GUI components are built. For instance, a call of the form:

from tkinter import *
widget = Button(text="Press me", command=someFunction)

creates a new button and specifies its text and callback function, using the text and command keyword arguments. Since the number of configuration options for a widget can be large, keyword arguments let you pick and choose which to apply. Without them, you might have to either list all the possible options by position or hope for a judicious positional argument defaults protocol that would handle every possible option arrangement.

Many built-in functions in Python expect us to use keywords for usage-mode options as well, which may or may not have defaults. As we learned in Chapter 8, for instance, the sorted built-in:

sorted(iterable, key=None, reverse=False)

expects us to pass an iterable object to be sorted, but also allows us to pass in optional keyword arguments to specify a dictionary sort key and a reversal flag, which default to None and False, respectively. Since we normally don’t use these options, they may be omitted to use defaults.