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In addition to knowing about namespaces, there are some other important terms to know about when installing and working with modules:

• https://pypi.python.org/pypi is the primary database for third-party Python packages.
• pip is the primary installer program for third-party modules and, since Python 3.4, has been included by default with Python binary installations.
• A virtual Python environment allows packages to be installed for a particular application's development, rather than being installed system-wide.
• venv has been the primary tool for creating virtual Python environments since Python 3.3. With Python 3.4, it automatically installs pip and setuptools in all virtual environments.
• The following are common terms for Python files: module, package, library, and distribution. While they have distinct definitions (https://packaging.python.org/glossary/), this book will use them interchangeably at times.

The following is part of dice_roller.py, an example of embedded tests from one of the first Python programs this author wrote when first learning Python:

import random
def randomNumGen(choice):
    if choice == 1: #d6 roll
        die = random.randint(1, 6)
    elif choice == 2: #d10 roll
        die = random.randint(1, 10)
    elif choice == 3: #d100 roll
        die = random.randint(1, 100)
    elif choice == 4: #d4 roll
      die = random.randint(1, 4)
    elif choice == 5: #d8 roll
      die = random.randint(1, 8)
    elif choice == 6: #d12 roll
      die = random.randint(1, 12)
    elif choice == 7: #d20 roll
      die = random.randint(1, 20)
    else: #simple error message
        return "Shouldn't be here. Invalid choice"
    return die
if __name__ == "__main__":
    import sys
    print(randomNumGen(int(sys.argv[1])))

In this example, we are simply creating a random number generator that simulates rolling different polyhedral dice (commonly used in role-playing games). The random library is imported, then the function defining how the dice rolls are generated is created. For each die roll, the integer provided indicates how many sides the die has. With this method, any number of possible values can be simulated with a single integer input.

The key part of this program is at the end. The part if __name__ == "__main__" tells Python that, if the namespace for the module is main, that is, it is the main program and not imported into another program, then the interpreter should run the code below this line. Otherwise, when imported, only the code above this line is available to the main program. (It's also worth noting that this line is necessary for cross-platform compatibility with Windows.)

When this program is called from the command line, the sys library is imported. Then, the first argument provided to the program is read from the command line and passed into the randomNumGen() function as an argument. The result is printed to the screen. Following are some examples of results from this program:

$ python3 dice_roller.py 1
2
$ python3 dice_roller.py 2
10
$ python3 dice_roller.py 3
63
$ python3 dice_roller.py 4
2
$ python3 dice_roller.py 5
5
$ python3 dice_roller.py 6
6
$ python3 dice_roller.py 7
17
$ python3 dice_roller.py 8
Shouldn't be here. Invalid choice

Configuring a module in this manner is an easy way to allow a user to interface directly with the module on a stand-alone basis. It is also a great way to run tests on the script; the tests are only run when the file is called as a stand-alone, otherwise the tests are ignored. dice_roller_tests.py is the full dice-rolling simulator that this author wrote:

import random #randint
def randomNumGen(choice):
    """Get a random number to simulate a d6, d10, or d100 roll."""
    
    if choice == 1: #d6 roll
      die = random.randint(1, 6)
    elif choice == 2: #d10 roll
        die = random.randint(1, 10)
    elif choice == 3: #d100 roll
        die = random.randint(1, 100)
    elif choice == 4: #d4 roll
      die = random.randint(1, 4)
    elif choice == 5: #d8 roll
      die = random.randint(1, 8)
    elif choice == 6: #d12 roll
      die = random.randint(1, 12)
    elif choice == 7: #d20 roll
      die = random.randint(1, 20)
    else: #simple error message
        return "Shouldn't be here. Invalid choice"
    return die
def multiDie(dice_number, die_type):
    """Add die rolls together, e.g. 2d6, 4d10, etc."""
    
#---Initialize variables 
    final_roll = 0
    val = 0
    
    while val < dice_number:
        final_roll += randomNumGen(die_type)
        val += 1
    return final_roll
def test():
    """Test criteria to show script works."""
    
    _1d6 = multiDie(1,1) #1d6
    print("1d6 = ", _1d6, end=' ') 
    _2d6 = multiDie(2,1) #2d6
    print("\n2d6 = ", _2d6, end=' ')
    _3d6 = multiDie(3,1) #3d6
    print("\n3d6 = ", _3d6, end=' ')
    _4d6 = multiDie(4,1) #4d6
    print("\n4d6 = ", _4d6, end=' ')
    _1d10 = multiDie(1,2) #1d10
    print("\n1d10 = ", _1d10, end=' ')
    _2d10 = multiDie(2,2) #2d10
    print("\n2d10 = ", _2d10, end=' ')
    _3d10 = multiDie(2,2) #3d10
    print("\n3d10 = ", _3d10, end=' ')
    _d100 = multiDie(1,3) #d100
    print("\n1d100 = ", _d100, end=' ') 
    
if __name__ == "__main__": #run test() if calling as a separate program
    test()

This program builds on the previous random-dice program by allowing multiple dice to be added together. In addition, the test() function only runs when the program is called by itself to provide a sanity check of the code. The test function would probably be better if it wasn't in a function with the rest of the code, as it is still accessible when the module is imported, as shown below:

>>> import dice_roller_tests.py
>>> dice_roller_tests.test()
1d6 = 1 
2d6 = 8 
3d6 = 10 
4d6 = 12 
1d10 = 5 
2d10 = 8 
3d10 = 6 
1d100 = 26

So, if you have any code you don't want to be accessible when the module is imported, make sure to include it below the line, as it were.