For loops

There are two ways to create loops in Python: with the for-loop and the while-loop.

When do I use for loops

for loops are used when you have a block of code which you want to repeat a fixed number of times. The for-loop is always used in combination with an iterable object, like a list or a range. The Python for statement iterates over the members of a sequence in order, executing the block each time. Contrast the for statement with the ''while'' loop, used when a condition needs to be checked each iteration or to repeat a block of code forever. For example:

For loop from 0 to 2, therefore running 3 times.

for x in range(0, 3):
    print("We're on time %d" % (x))

While loop from 1 to infinity, therefore running forever.

x = 1
while True:
    print("To infinity and beyond! We're getting close, on %d now!" % (x))
    x += 1

When running the above example, you can stop the program by pressing ctrl+c at the same time. As you can see, these loop constructs serve different purposes. The for loop runs for a fixed amount of times, while the while loop runs until the loop condition changes. In this example, the condition is the boolean True which will never change, so it will run forever.

How do they work?

If you've done any programming before, you have undoubtedly come across a for loop or an equivalent to it. Many languages have conditions in the syntax of their for loop, such as a relational expression to determine if the loop is done, and an increment expression to determine the next loop value. In Python, this is controlled instead by generating the appropriate sequence. Basically, any object with an iterable method can be used in a for loop. Even strings, despite not having an iterable method - but we'll not get on to that here. Having an iterable method basically means that the data can be presented in list form, where there are multiple values in an orderly fashion. You can define your own iterables by creating an object with next() and iter() methods. This means that you'll rarely be dealing with raw numbers when it comes to for loops in Python - great for just about anyone!

Nested loops

When you have a block of code you want to run x number of times, then a block of code within that code which you want to run y number of times, you use what is known as a "nested loop". In Python, these are heavily used whenever someone has a list of lists - an iterable object within an iterable object.

for x in range(1, 11):
    for y in range(1, 11):
        print('%d * %d = %d' % (x, y, x*y))

• Early exits

Like the while loop, the for loop can be made to exit before the given object is finished. This is done using the break statement, which will immediately drop out of the loop and continue execution at the first statement after the block. You can also have an optional else clause, which will run should the for loop exit cleanly - that is, without breaking.

for x in range(3):
    if x == 1:
        break

Examples

For..Else

for x in range(3):
    print(x)
else:
    print('Final x = %d' % (x))

Strings as an iterable

string = "Hello World"
for x in string:
    print(x)

Lists as an iterable

collection = ['hey', 5, 'd']
for x in collection:
    print(x)

Loop over Lists of lists

list_of_lists = [ [1, 2, 3], [4, 5, 6], [7, 8, 9]]
for list in list_of_lists:
    for x in list:
        print(x)

Creating your own iterable

class Iterable(object):

    def __init__(self,values):
        self.values = values
        self.location = 0

    def __iter__(self):
        return self

    def next(self):
        if self.location == len(self.values):
            raise StopIteration
        value = self.values[self.location]
        self.location += 1
        return value

Your own range generator using yield

def my_range(start, end, step):
    while start <= end:
        yield start
        start += step

for x in my_range(1, 10, 0.5):
    print(x)

A note on `range`

The ''range'' function is seen so often in for statements that you might think range is part of the for syntax. It is not: it is a Python built-in function that returns a sequence following a specific pattern (most often sequential integers), which thus meets the requirement of providing a sequence for the for statement to iterate over. Since for can operate directly on sequences, there is often no need to count. This is a common beginner construct (if they are coming from another language with different loop syntax):

mylist = ['a', 'b', 'c', 'd']
for i in range(len(mylist)):
    # do something with mylist[i]

It can be replaced with this:

mylist = ['a', 'b', 'c', 'd']
for v in mylist:
    # do something with v

Consider for var in range(len(something)): to be a flag for possibly non-optimal Python coding.

More resources

If you'd like to learn more, try these links:

• Python.org docs

• Python for loop and while loop tutorial with interactive code examples