View source for Advanced Python (Fall 2017)/lecture2

= Lecture 2 =

== Scope and style ==

Python scope is similar to other programming language and does not have many suprises. Basic scoping rules can be found here: https://en.wikipedia.org/wiki/Scope_(computer_science)#Python

<source lang="python">my_variable = 1  # in the global scope

def my_function():
    print(my_variable)


my_function()  # prints 1
print(my_variable)  # prints 1</source>
variables in the global scope are accessible in the local scope of a function

<source lang="python">def my_function():
    my_variable = 2  # in the function scope
    print(my_variable)


my_function()  # prints 2
print(my_variable)  # raises NameError</source>
variables defined in the local scope of a function are not available outside the function

<source lang="python">my_variable = 1  # in the global scope

def my_function():
    my_variable = 2  # in the function scope
    print(my_variable)


my_function()  # prints 2
print(my_variable)  # prints 1</source>
functions have there own scope and assignment statement in variables (i.e. <code>my_variable = 2</code> creates a new variable rather than using a variable in the global scope.

<source lang="python">my_variable = 1  # in the global scope

def my_function():
    global my_variable
    my_variable = 2  # in the function scope
    print(my_variable)


my_function()  # prints 2
print(my_variable)  # prints 1</source>
To modify a variable in the global scope from inside a function, you can use the <code>global</code> keyword. NB! Modifying global variables if bad practice and should not be done in an application you write!

=== Task 1 ===

Replace the use of global with something better.

<source lang="python">x = 0

def add_1():
    global x
    x += 1

add_1()
add_1()
add_1()

print(x)  # prints 3</source>
possible solution

<source lang="python">class MyCounter:
    def __init__(self, x):
        self.x = x

    def add_1(self):
        self.x += 1


my_counter = MyCounter(0)
my_counter.add_1()
my_counter.add_1()
my_counter.add_1()
print(my_counter.x)</source>
== Python Packaging (Part 1) ==

We will do another lesson on this.

=== Step 1 make a git repository ===

This is not required to make a python package. You can make a python package without <code>git</code>, but we will use <code>git</code> in this example.

In github, make a repositiory and clone the empty repository. Make sure there is a <code>.gitignore</code> with appropriate rules for python.

=== Step 2 add some necessary files ===

To make a python package, you need a python module and a file called <code>setup.py</code>. <code>setup.py</code> can start with this:

<source lang="python">from distutils.core import setup

setup()</source>
the function <code>setup</code> then takes keyword arguments that describe our project.

Now make a python module. I will use a module called <code>example</code>. So I will make a directory called <code>example</code> and put an empty file called <code>__init__.py</code> in the directory <code>example</code>

to make a file that will execute when we call this package from the command line, then create a file called <code>__main__.py</code>

=== step 3 add some code ===

you can add these keyword arguments to <code>setup</code> in <code>setup.py</code>

<source lang="python">from distutils.core import setup

setup(
    name='firstpackage',
    version='0.1',
    packages=['example'],
)</source>
<code>packages</code> should have the name of your python module in it.

add something to execute in <code>__main__.py</code> like:

<source lang="python">print('Hi from my first package')</source>
=== Using your new python package ===

You can build it with <code>python setup.py sdist</code>. This will create an archive in a newly created <code>dist</code> directory you can install it with <code>pip install dist/firstpackage-0.1.tar.gz</code> and you can run it with <code>python -m example</code>. now you will see 'Hi from my first package' printed in the terminal!