Creating a Python Module

To get started, let's run the using_modules.py file to see what our first issue is.

$ python3.7 using_modules.py
Traceback (most recent call last):
  File "using_modules.py", line 7, in <module>
    assert math.ceil(14.11) == 15, f"Expected 15, but got {math.ceil(14.11)}"
NameError: name 'math' is not defined

We're attempting to use the math module, but it has not been imported. Let's import the math module now.

~/using_modules.py

# 1) Import the built-in `math` module
import math

# 2) Import the `reverse` and `str_shuffle` as `shuffle` from the custom `strhelpers` module (Needs to be created)

name = "Kevin Bacon"

assert math.ceil(14.11) == 15, f"Expected 15, but got {math.ceil(14.11)}"
assert (
    reverse(name) == "nocaB niveK"
), f"Expected 'nocaB niveK', but got {reverse(name)}"
assert type(shuffle(name)) == str, f"Expected a string, but got {type(shuffle(name))}"
assert sorted(shuffle(name)) == sorted(
    name
), f"Expected [' ', 'B', 'K', 'a', 'c', 'e', 'i', 'n', 'n', 'o', 'v'], but got {sorted(shuffle(name))}"

We've imported the whole math module, so now we should be able to run the file and see a different error.

$ python3.7 using_modules.py
Traceback (most recent call last):
  File "using_modules.py", line 10, in <module>
    reverse(name) == "nocaB niveK"
NameError: name 'reverse' is not defined

We're going to be creating a module called strhelpers that will contain two functions we need: reverse and str_shuffle. For now, we're going to just focus on the reverse function. From the previous error, we saw this function needs to take a string and return that string in reverse. Let's create our module by creating a strhelper.py file right next to the using_modules.py file, and then define a reverse function.

~/strhelpers.py

def reverse(str_value):
    return str_value[::-1]

We've used string slicing with a negative step value to reverse the string. Now we need to import this function from this module using a from ... import statement.

~/using_modules.py

# 1) Import the built-in `math` module
import math

# 2) Import the `reverse` and `str_shuffle` as `shuffle` from the custom `strhelpers` module (Needs to be created)
from strhelpers import reverse

name = "Kevin Bacon"

assert math.ceil(14.11) == 15, f"Expected 15, but got {math.ceil(14.11)}"
assert (
    reverse(name) == "nocaB niveK"
), f"Expected 'nocaB niveK', but got {reverse(name)}"
assert type(shuffle(name)) == str, f"Expected a string, but got {type(shuffle(name))}"
assert sorted(shuffle(name)) == sorted(
    name
), f"Expected [' ', 'B', 'K', 'a', 'c', 'e', 'i', 'n', 'n', 'o', 'v'], but got {sorted(shuffle(name))}"

Running this again, we should see a new error.

$ python3.7 using_modules.py
Traceback (most recent call last):
  File "using_modules.py", line 13, in <module>
    assert type(shuffle(name)) == str, f"Expected a string, but got {type(shuffle(name))}"
NameError: name 'shuffle' is not defined

Our final task is to implement a function called str_shuffle in the strhelpers module. This function needs to take a string and return a shuffled version of it. Thankfully, the standard library includes a way to shuffle a list, and we can convert strings to lists and back again. Let's implement the str_shuffle function using the random.shuffle function.

~/strhelpers.py

from random import shuffle as l_shuffle

def reverse(str_value):
    return str_value[::-1]

def str_shuffle(str_value):
    str_list = list(str_value)
    l_shuffle(str_list)
    return "".join(str_list)

A thing to note here is that random.shuffle doesn't return a new list. It modifies the existing list. Next, let's import this function into using_modules.py, giving it the identifier shuffle.

~/using_modules.py

# 1) Import the built-in `math` module
import math

# 2) Import the `reverse` and `str_shuffle` as `shuffle` from the custom `strhelpers` module (Needs to be created)
from strhelpers import reverse, str_shuffle as shuffle

name = "Kevin Bacon"

assert math.ceil(14.11) == 15, f"Expected 15, but got {math.ceil(14.11)}"
assert (
    reverse(name) == "nocaB niveK"
), f"Expected 'nocaB niveK', but got {reverse(name)}"
assert type(shuffle(name)) == str, f"Expected a string, but got {type(shuffle(name))}"
assert sorted(shuffle(name)) == sorted(
    name
), f"Expected [' ', 'B', 'K', 'a', 'c', 'e', 'i', 'n', 'n', 'o', 'v'], but got {sorted(shuffle(name))}"

Finally, let's run using_modules.py one last time to ensure there are no more errors.

$ python3.7 using_modules.py
$