Functional-Style Programming in Java

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sions (or list comprehensions) and map() and filter() is most often purely a matter of personal programming style Using map(), filter(), and functoolsreduce() often leads to the elimination of loops, as the examples we have seen illustrate These functions are useful when converting code written in a functional language, but in Python we can usually replace map() with a list comprehension and filter() with a list comprehension with a condition, and many cases of functoolsreduce() can be eliminated by using one of Python s built-in functional functions such as all(), any(), max(), min(), and sum() For example:
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sum(ospathgetsize(x) for x in files if xendswith("py"))
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This achieves the same thing as the previous three examples, but is much more compact
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operator attrgetter()
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In addition to providing functions for Python s operators, the operator module also provides the operatorattrgetter() and operatoritemgetter() functions, the rst of which we brie y met earlier in this chapter Both of these return functions which can then be called to extract the speci ed attributes or items Whereas slicing can be used to extract a sequence of part of a list, and slicing with striding can be used to extract a sequence of parts (say, every third item with L[::3]), operatoritemgetter() can be used to extract a sequence of arbitrary parts, for example, operatoritemgetter(4, 5, 6, 11, 18)(L) The function returned by operatoritemgetter() does not have to be called immediately and thrown away as we have done here; it could be kept and passed as the function argument to map(), filter(), or functoolsreduce(), or used in a dictionary, list, or set comprehension When we want to sort we can specify a key function This function can be any function, for example, a lambda function, a built-in function or method (such as strlower()), or a function returned by operatorattrgetter() For example, assuming list L holds objects with a priority attribute, we can sort the list into priority order like this: Lsort(key=operatorattrgetter("priority")) In addition to the functools and operator modules already mentioned, the itertools module can also be useful for functional-style programming For example, although it is possible to iterate over two or more lists by concatenating them, an alternative is to use itertoolschain() like this:
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for value in itertoolschain(data_list1, data_list2, data_list3): total += value
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The itertoolschain() function returns an iterator that gives successive values from the rst sequence it is given, then successive values from the second sequence, and so on until all the values from all the sequences are used The itertools module has many other functions, and its documentation gives many
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small yet useful examples and is well worth reading (Note also that a couple of new functions were added to the itertools module with Python 31)
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Partial Function Application
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Partial function application is the creation of a function from an existing function and some arguments to produce a new function that does what the original function did, but with some arguments xed so that callers don t have to pass them Here s a very simple example:
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enumerate1 = functoolspartial(enumerate, start=1) for lino, line in enumerate1(lines): process_line(i, line)
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The rst line creates a new function, enumerate1(), that wraps the given function (enumerate()) and a keyword argument (start=1) so that when enumerate1() is called it calls the original function with the xed argument and with any other arguments that are given at the time it is called, in this case lines Here we have used the enumerate1() function to provide conventional line counting starting from line 1 Using partial function application can simplify our code, especially when we want to call the same functions with the same arguments again and again For example, instead of specifying the mode and encoding arguments every time we call open() to process UTF-8 encoded text les, we could create a couple of functions with these arguments xed:
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reader = functoolspartial(open, mode="rt", encoding="utf8") writer = functoolspartial(open, mode="wt", encoding="utf8")
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Now we can open text les for reading by calling reader(filename) and for writing by calling writer(filename) One very common use case for partial function application is in GUI (Graphical User Interface) programming (covered in 15), where it is often convenient to have one particular function called when any one of a set of buttons is pressed For example:
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loadButton = tkinterButton(frame, text="Load", command=functoolspartial(doAction, "load")) saveButton = tkinterButton(frame, text="Save", command=functoolspartial(doAction, "save"))
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This example uses the tkinter GUI library that comes as standard with Python The tkinterButton class is used for buttons here we have created two, both contained inside the same frame, and each with a text that indicates its purpose Each button s command argument is set to the function that tkinter
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