global_&_return.rst

Global and Return

You might have encountered some functions written in Python which have a return keyword in the end of the function. Do you know what it does? It is similar to return in other languages. Let's examine this little function:

def add(value1, value2):
    return value1 + value2

result = add(3, 5)
print(result)
# Output: 8

The function above takes two values as input and then outputs their addition. We could have also done:

def add(value1,value2):
    global result
    result = value1 + value2

add(3,5)
print(result)
# Output: 8

So first let's talk about the first bit of code which involves the return keyword. What that function is doing is that it is assigning the value to the variable which is calling that function which in our case is result. In most cases you won't need to use the global keyword. However, let's examine the other bit of code as well which includes the global keyword. So what that function is doing is that it is making a global variable result. What does global mean here? Global variable means that we can access that variable outside the scope of the function as well. Let me demonstrate it with an example:

# first without the global variable
def add(value1, value2):
    result = value1 + value2

add(2, 4)
print(result)

# Oh crap, we encountered an exception. Why is it so?
# the python interpreter is telling us that we do not
# have any variable with the name of result. It is so
# because the result variable is only accessible inside
# the function in which it is created if it is not global.
Traceback (most recent call last):
  File "", line 1, in
    result
NameError: name 'result' is not defined

# Now lets run the same code but after making the result
# variable global
def add(value1, value2):
    global result
    result = value1 + value2

add(2, 4)
result
6

So hopefully there are no errors in the second run as expected. In practical programming you should try to stay away from global keyword as it only makes life difficult by introducing unwanted variables to the global scope.

Multiple return values

So what if you want to return two variables from a function instead of one? There are a couple of approaches which new programmers take. The most famous approach is to use global keyword. Let's take a look at a useless example:

def profile():
    global name
    global age
    name = "Danny"
    age = 30

profile()
print(name)
# Output: Danny

print(age)
# Output: 30

**Note:**Don't try to use the above mentioned method. I repeat, don't try to use the above mentioned method!

Some try to solve this problem by returning a tuple, list or dict with the required values after the function terminates. It is one way to do it and works like a charm:

def profile():
    name = "Danny"
    age = 30
    return (name, age)

profile_data = profile()
print(profile_data[0])
# Output: Danny

print(profile_data[1])
# Output: 30

Or by more common convention:

def profile():
    name = "Danny"
    age = 30
    return name, age

profile_name, profile_age = profile()
print(profile_name)
# Output: Danny
print(profile_age)
# Output: 30

Keep in mind that even in the above example we are returning a tuple (despite the lack of parenthesis) and not separate multiple values. If you want to take it one step further, you can also make use of namedtuple. Here is an example:

from collections import namedtuple
def profile():
    Person = namedtuple('Person', 'name age')
    return Person(name="Danny", age=31)

# Use as namedtuple
p = profile()
print(p, type(p))
# Person(name='Danny', age=31) <class '__main__.Person'>
print(p.name)
# Danny
print(p.age)
#31

# Use as plain tuple
p = profile()
print(p[0])
# Danny
print(p[1])
#31

# Unpack it immediatly
name, age = profile()
print(name)
# Danny
print(age)
#31

This is a better way to do it, along with returning list and dict. Don't use global keyword unless you know what you are doing. global might be a better option in a few cases but is not in most of them.