Revisiting Python: Functions

This is the second post in the series on revisiting Python. In the first post we discussed the motivation for these posts and started by revisiting Built-in Types.

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In this post, we’ll talk about functions in Python 2.7.

Functions

Functions in python consist of the keyword def, a function name and a list of arguments:

def hello(arg1, arg2):
  print "hello world"

The pythonic-way to document what a function does is through docstrings (block of strings delimited by triple-double quotes:

def hello(arg1, arg2):
  """
    Prints hello world
  """
  print "hello world"

Some tools are able to parse these docstrings to automatically build documentation.

Namespace. is a map from names (variables names, function names) to objects (their values). Examples of namespaces include the built-in namespace, the global namespace within a module and local namespace within a function.

Scope. is a textual region of a Python program where a namespace is directly accessible.

For example, within a function we have a local scope. Variable from the arguments or created within the function itself belong to this local scope. Consider the following example:

d = 4
def foo(a):
    b = 1
    return abs(a - b)
foo(10)

In the code above, a, b are in the local scope. d is in the module global scope and the function abs is in the built-in scope. When we refer to a variable or function name, Python search scopes in the following order:

1. Local scope
2. Local scope of the calling functions
3. Global scope within a module
4. Built-in scope

Item 2 requires some clarification. In Python we can define a function inside another function. For example,

def outer():
    scope_name = 'outer'
    def inner():
        print(scope_name)
    return inner

scope_name = 'global'
# Get the inner function
inner = outer()
inner() # outer

In the code above, scope_name is defined both in the global scope and in the outer() function scope. When referenced within inner(), the outer scope takes precedence, so this will print 'outer'. Note how the scope is resolved statically, when defining inner(), not in runtime, explaining why the docs emphasize “textual region“. This is known as lexical scoping, as we discussed when studying the R language.

Variables that are not in the local scope are read-only by default. If we try to assign a new value to an existing variable in an outer scope, we are just creating a new local variable with the same name. Note that if the global variable is a reference to an object, we can modify the object, but we still can’t re-assign a value to that global variable.

x = [1, 2, 3]

def modify_global_variable_content():
    x.append(4) 

def reassign_global_variable():
    x = [10, 20, 30]

modify_global_variable_content()
reassign_global_variable()
print x

In the code above, modify_global_variable_content() modifies the content of x, but reassign_global_variable() just creates a local variable x. To actually modify the variable we can make it explicit using the global keyword.

def reassign_global_variable():
    global x
    x = [10, 20, 30]

Functions can be assigned to variables and passed around, for example:

def inc(x):
    return x + 1

def print_f(f, x):
    print f(x)

print_f(inc, 2)

Here we pass the function inc(), which then is assigned to the variable f in print_f(), and used to invoke the original function.

Default arguments. It’s possible to define default arguments to functions, like in C++. The initialization only happens once and the value is shared between subsequent calls, so for objects, this is what happens:

def append(x, xs = []):
    xs.append(x)
    return xs

print append(1) # [1]
print append(2) # [1, 2]
print append(3) # [1, 2, 3]

Keyword arguments. The default match between the arguments passed to a function and the arguments the function receives is positional, that is, the first value is assigned to the first parameter and so on. Besides that, we can make the argument names explicit.

def listify(a, b, c):
    return [a, b, c]
print listify(b = 2, c = 3, a = 1)

There are some constraints though: 1. Positional arguments must come before keyword arguments. 2. If we use a keyword for a given argument, we have to name all required arguments that appear after it in the function definition.

def listify(a, b, c):
    return [a, b, c]

# Error: Violates rule 1
print listify(c = 3, 1, 2)

# Error: Violates rule 2
print listify(2, 3, a = 1)

This feature is one of my favorite in Python, which I miss from many other languages. This is particular useful in the scenario where a function takes many default arguments, but we only want to provide the last one.

Var arg. Functions can define two different sets of variable arguments. An argument named * is assigned with a list of all position arguments passed beyond the required arguments; an argument named ** is assigned with a dictionary with all keyword arguments not in the required arguments. A simple example:

def f(fixed, *arguments, **keywords):
    print fixed     # 1
    print arguments # (2, 3) 
    print keywords  # {a: 4, b: 5}

f(1, 2, 3, a = 4, b = 5)

Argument unpacking. With an analogous idea, it’s possible to call a function with a list of arguments using the * modifier. For example:

def f(a, b, c):
    print a, b, c
args = [1, 2, 3] 
f(*args)

Or a map of keyword arguments using the ** modifier:

def f(a, b, c):
    print a, b, c
args = {'b': 2, 'c': 3, 'a': 1}
f(**args)

Generators

In the previous post we saw the iterator type. Remember how to create a custom iterator, he had to define a class and implement the required methods (__iter__() and next()).

We can also instantiate a generator from a function, by using the yield keyword. For example,

def sample_generator(n):
    for i in range(n):
        yield i

The result of the function is an iterator. Every time we run next(), we execute the function until it reaches a yield statement, in which case it return that value.

it = sample_generator(10)
it.next() # 0
it.next() # 1
...

Generators are particularly useful if the iterator has to work in sequential steps that performs different actions based at each step. For example, consider the following iterator:

class MyIterator:
    def __init__(self, x):
        self.step = 0
        self.x = x

    def __iter__(self):
        return self

    def next(self):
        if self.step == 0:
            ret = self.x
        elif self.step == 1:
            ret = 2*self.x
        elif self.step == 2:
            ret = self.x*self.x
        else:
            raise StopIteration

        self.step += 1
        return ret

This is a toy example that runs different functions of x based on which stage it is on. Using generators, this would simply become:

def my_generator(x):
    yield x
    yield 2*x
    yield x*x

A generator can use the return statement (without value) to stop the iterator, which is equivalent to raise StopIteration.

It’s possible to send values back to generators through the method send(). Consider the following example:

def generator_with_sends(x):
    while True:
        y = (yield x)
        if y is not None:
            x = y

it = generator_with_sends(5)
print it.next()   # 5
print it.send(10) # 10
print it.next()   # 10

Note that it.send(10) returns 10, not 5. This is because yield halts when the right hand side of the statement is evaluated but before the assignment. Calling send() will resume from there, but this time the return value of (yield x) will be the value passed to send. The code will then execute until the next yield, in which x contains the value passed to y. It’s not allowed to call send() before the generator had yielded the first time.

When a generator is garbage-collected it raises an GeneratorExit exception. It’s possible to force that by calling the stop() method.

def stubborn_generator():
    try:
        while True:
            yield 'no'
    except GeneratorExit:
	print 'generator stopped. clean up...'
        raise StopIteration
    finally:
        print 'can do some shared clean up here'

def scoped():
    s = stubborn_generator()
    print s.next()

scoped()

Map, filter, reduce

Python has built-in function that work with sequences: map(), filter() and reduce() being the most common. These functions work with iterators too. For example:

# map_generator.py
def generate_ints(n):
    for i in range(n):
        yield i

def double(x):
    return x*2

print map(double, generate_ints(5)) 
# [0, 2, 4, 6, 8]

The problem is that it has to convert a generator to a list, so it has to evaluate a generator until it stops, thus we can’t do this with an “infinite” iterator, like the one from this generator:

def generate_forever():
    i = 0
    while True:
        yield i
        i += 1

the alternative is using the itertools library, which contains the imap() and ifilter() methods, which generate new iterators with the function applied.

h = itertools.imap(double, generate_forever())
for i in range(10):
    print h.next()

We can see generators as lazily evaluated functions, which is the default behavior in languages like Haskell.

Partial function application

One very useful functional paradigm is partial application. It’s available in the functools module. For example, we can re-write our double() (from map_generator.py) function as follows:

# partial.py
import operator
import functools

double = functools.partial(operator.mul, 2)

Here, operator.mul is just the function version for the * operator, that is,

def mul(a, b):
    return a*b

The code partial.py will return another function where the first argument to the operator.mul function is replaced by 2. The nice thing about the partial() function is that it accepts keyword arguments:

def f(a, b, c, d):
    return [a, b, c, d]
g = functools.partial(f, b=2, c=3)
print g(a=1, d=4)

Conclusion

In this second post we covered functions in Python. We learned about namespaces, scopes and variable number of arguments. We also learned about generators and increased our knowledge on iterators by introducing the itertools module. Finally, we saw that Python can work with higher-order functions by using libraries like functools.

References

[1] The Python Tutorial: More Control Flow Tools
[2] The Python HOWTOs: Functional Programming HOWTO

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