Even Number of Customers |2|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In this question, the task is for helping a shop owner who wants to know
whether there are even number of customers in her shop at any time.

Whenever she needs the answer, she should call *even_customers* function
with no argument and this function must return ``True`` or ``False``
according to the number of customers in the shop being even or odd
respectively. To keep track of the incoming and outgoing customers, she
will call *incoming* and *outgoing* functions. These functions do not
take any arguments and do not return anything.

*Hint:* Think about using a ``global`` variable which is accessible to
all three functions.

.. container:: sampleio

   Sample I/O:

.. |2| image:: ../../figures/difficulty_three.png
   :class: difficulty

.. code:: default

   >>> from even_customers import *
   >>> incoming()
   >>> incoming()
   >>> incoming()
   >>> even_customers()
   False
   >>> outgoing()
   >>> even_customers()
   True

In these sample calls, as 3 consecutive calls of *incoming* function
shows, 3 customers arrived to the shop. That’s why the first call of
*even_customers* function returns ``False``. Then a customer left, as
indicated by the call of *outgoing* function. At that time, there were 2
customers in the shop, therefore, the second call of *even_customers*
returns ``True``.

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The solution will be constructed incrementally with requested functions
and a common, global variable. First of all, let’s define this global
variable:

.. code:: python

   counter = 0

Since the definition of this variable is not in any function, it will be
in the global scope. Therefore, all of the functions will be able to
reach this variable.

Now we can define *incoming* function as below:

.. code:: python

   def incoming():
       global counter
       counter += 1

This function needs to update global variable *counter*, and to be able
to do that, it is declared with global keyword at the beginning of
function.

The *outgoing* function can be defined in the same manner:

.. code:: python

   def outgoing():
       global counter
       if counter > 0: counter -= 1

Finally, the definition of *even_customers* function becomes like
following:

.. code:: python

   def even_customers():
       return counter % 2 == 0

Note that the global variable, *counter* is used read-only and not
updated here. Therefore declaration with global keyword is not
necessary.

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