More Python features

To finish off this course, here you will find various useful Python features.

Single line conditionals

The following two statements produce the exact same results:

if x%2 == 0:
    print("even")
else:
    print("odd")

print("even" if x%2 == 0 else "odd")

In the latter example we have a conditional expression on a single line: a if [condition] else b. The value of this expression evaluates to a if the condition is true, and b if it is false. This structure is sometimes referred to as a ternary operator.

Conditional expressions can be very useful when you need to assign something conditionally. For example, if you had the variables x and y, and you wanted to either increment or set the value of y depending on the parity of x, you could write it in a normal if else statement, like so:

if x%2 == 0:
    y += 1
else:
    y = 0

The same could be achieved with a nifty one-liner:

y = y + 1 if x%2 == 0 else 0

An "empty" block

You may remember from the previous part that you are not allowed to have an empty block in a Python program. If you need to have a block of code which does nothing, for example when testing some other functionality, the pass command will let you do this. You could, for instance, write a function which does nothing:

def testing():
    pass

This function would simply return immediately. Leaving the pass command out, i.e. having a completely empty block, would produce an error.

def testing():  # this causes an error!

Loops with else blocks

In Python, loops can have else blocks, too. This section of code is executed if the loop finishes normally.

For example, in the following example we are looking through a list of numbers. If there is an even number on the list, the program prints out a message and the loop is broken. If there are no even numbers, the loop finishes normally, but a different message is then printed out.

my_list = [3,5,2,8,1]
for x in my_list:
    if x%2 == 0:
        print("found an even number", x)
        break
else:
    print("there were no even numbers")

A more traditional way to achieve this would be to use a helper variable to remember whether the desired item was found:

my_list = [3,5,2,8,1]
found = False
for x in my_list:
    if x%2 == 0:
        print("found an even number", x)
        found = True
        break
if not found:
    print("there were no even numbers")

Using a for else statement saves us the trouble of writing a separate variable.

Default parameter value

A Python function can have a default parameter value. It is used whenever no argument is passed to the function. See the following example:

def say_hello(name="Emily"):
    print("Hi there,", name)

say_hello()
say_hello("Eric")
say_hello("Matthew")
say_hello("")

NB: an empty string is still a string, so the default parameter is not used if an empty string is passed to the function.

A variable number of parameters

You can also define a function with a variable number of parameters, by adding a star before the parameter name. All the remaining arguments passed to the function are contained in a tuple, and can be accessed through the named parameter.

The following function counts the number and sum of its arguments:

def testing(*my_args):
    print("You passed", len(my_args), "arguments")
    print("The sum of the arguments is", sum(my_args))

testing(1, 2, 3, 4, 5)

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