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# Developing Modular Code
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===
## What is modularity?
Simple components combined to produce complex behavior.
- Software can be 'built up' from smaller elements.
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- Elements are self-contained and independent.
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- Each element handles a specific (set of) task(s).
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<figure style="text-align: center; margin-top: 20px;">
<img height="300" alt="modularity" src="./media/modular-code/modern_times.jpg" style="margin-bottom: -10px;">
<figcaption style="font-size: 0.3em; color: #666;">Charlie Chaplin in <i>Modern Times</i> (1936).<br>© Roy Export Company Establishment; photograph, the Museum of Modern Art/Film Stills Archive, New York City</figcaption>
</figure>
===
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## Modular code
<div style="display: flex; justify-content: center; align-items: center; margin-top: 20px;">
<img width="400" alt="cohesive" src="./media/modular-code/small_cohesive_units.png" style="margin-right: 20px;">
<img width="400" alt="bohemoth" src="./media/modular-code/customized_bohemoth.png" class="fragment" data-fragment-index="1">
</div>
===
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## What are these blocks/elements?
<div style="display: flex; justify-content: center; align-items: flex-start; margin-top: 20px; max-width: 100%;">
<img src="./media/modular-code/building_blocks.png" alt="building blocks" style="max-width: 40%; height: auto; margin-right: 20px;">
<div>
<ul>
<li class="fragment" data-fragment-index="1">functions</li>
<li class="fragment" data-fragment-index="2">classes</li>
<li class="fragment" data-fragment-index="3">modules</li>
<li class="fragment" data-fragment-index="4">libraries/packages</li>
<li class="fragment" data-fragment-index="5">programs</li>
<li class="fragment" data-fragment-index="5">...</li>
</ul>
</div>
</div>
===
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## Why write modular code?
To increase robustness
<img width="200" alt="testing a single module" src="./media/modular-code/testing_module.png">
- A well-designed module can be tested.
- This helps keep the codebase well-functioning and bug-free.
===
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## Why write modular code?
To make maintenance easier:
<img width="300" alt="testing a module taken from a larger project" src="./media/modular-code/testing_module_maintenance.png">
- Modular code is more readable and understandable.
- Modules can be debugged separately.
- Modules only need to be improved/optimized once.
===
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## Why write modular code?
To allow reusability:
<img width="400" alt="reuse a module in another project" src="./media/modular-code/reuse_module.png">
- A module can live independent of its original context.
- It can be reused by another project.
===
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## Why write modular code?
To facilitate scalability:
<div style="display: flex; align-items: center;">
<img alt="scalability" src="./media/modular-code/scalability.png" style="max-width: 40%; height: auto; margin-right: 20px;">
<div>
<ul>
<li>Complexity remains low by design.</li>
<li>This creates space for scaling up.</li>
</ul>
</div>
</div>
===
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## Why write modular code?
To create opportunities for innovation:
<img alt="tetris shows innovation" src="./media/modular-code/tetris_innovation.png" style="max-width: 50%; height: auto;">
- Modules increase the capabilities and power of a project.
- Rearrange old modules for new applications.
===
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## Why write modular code?
To save time:
<img width="600" alt="development speed" src="./media/modular-code/development-speed.svg">
===
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## A good module...
<div style="display: flex; justify-content: center; align-items: flex-start; margin-top: 20px;">
<img src="./media/modular-code/units.png" alt="units" width="300" style="margin-right: 20px;">
<div>
<ul>
<li class="fragment" data-fragment-index="1">performs limited and clearly defined tasks</li>
<li class="fragment" data-fragment-index="2">has a good name</li>
</ul>
</div>
</div>
===
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## Naming your module
- be descriptive and clear
- focus on human intelligibility
- follow language specific conventions
- avoid abbreviations
<div style="text-align: center; margin-top: 20px;">
<img alt="naming" src="./media/modular-code/naming_stuff.jpg" style="max-width: 100%; height: auto; max-height: 300px;">
</div>
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===
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## A good module...
<div style="display: flex; justify-content: center; align-items: flex-start; margin-top: 20px;">
<img src="./media/modular-code/units.png" alt="units" width="300" style="margin-right: 20px;">
<div>
<ul>
<li>performs limited and clearly defined tasks</li>
<li>has a good name</li>
<li class="fragment" data-fragment-index="1">is readable</li>
</ul>
</div>
</div>
===
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## More readable != shorter code
Shorter:
```python
indexATG = [n for n,i in enumerate(myList) if i == 'ATG']
indexAAG = [n for n,i in enumerate(myList) if i == 'AAG']
```
More modular:
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```python
def getIndex(inputList,z):
zIndex = [n for n,i in enumerate(inputList) if i == z]
return zIndex
indexATG = getIndex(myList,'ATG')
indexAAG = getIndex(myList,'AAG')
```
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===
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## A good module...
<div style="display: flex; justify-content: center; align-items: flex-start; margin-top: 20px;">
<img src="./media/modular-code/units.png" alt="units" width="300" style="margin-right: 20px;">
<div>
<ul>
<li>performs limited and clearly defined tasks</li>
<li>has a good name</li>
<li>is readable</li>
<li class="fragment" data-fragment-index="1">is pure and predictable</li>
</ul>
</div>
</div>
===
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## An impure function...
... does not always give the same result:
```python
def fahrenheit_to_celsius(temp_list, converted_temps=[]):
for temp in temp_list:
temp_c = (temp - 32.0) * (5.0/9.0)
converted_temps.append(temp_c)
return converted_temps
>>> fahrenheit_to_celsius([32.0, 77.0]) # [0.0, 25.0]
```
**Q: What will happen when we call the function a second time?**
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```python
>>> fahrenheit_to_celsius([32.0, 77.0]) # [0.0, 25.0, 0.0, 25.0]
```
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===
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## A pure function...
... always gives the same result:
```python
def fahrenheit_to_celsius(temp_list, converted_temps=None):
if converted_temps == None:
converted_temps = []
for temp in temp_list:
temp_c = (temp - 32.0) * (5.0/9.0)
converted_temps.append(temp_c)
return converted_temps
>>> fahrenheit_to_celsius([32.0, 77.0]) # [0.0, 25.0]
>>> fahrenheit_to_celsius([32.0, 77.0]) # [0.0, 25.0]
```
===
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## A good module...
<div style="display: flex; justify-content: center; align-items: flex-start; margin-top: 20px;">
<img src="./media/modular-code/units.png" alt="units" width="300" style="margin-right: 20px;">
<div>
<ul>
<li>performs limited and clearly defined tasks</li>
<li>has a good name</li>
<li>is readable</li>
<li>is pure and predictable</li>
</ul>
</div>
</div>
===
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## Identifying opportunities for modularization
<div style="display: flex; justify-content: center; align-items: flex-start; margin-top: 20px;">
<img src="./media/modular-code/testing_module.png" alt="testing a single module" width="300" style="margin-right: 20px;">
<div>
<ul>
<li class="fragment" data-fragment-index="2">poor readability</li>
</ul>
</div>
</div>
===
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## Readable code
Modular code becomes more readable
- Code is read more than it is written
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- Does a reader understand what the code does?
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- Bad readability can be a "code smell"
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<figure style="text-align: center; margin-top: 20px;">
<img width="400" alt="smelly" src="./media/modular-code/Smelly-Code-Featured.avif" style="margin-bottom: -10px;">
<figcaption style="font-size: 0.3em; color: #666;">SIphotography/Depositphotos</figcaption>
</figure>
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===
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## Identifying opportunities for modularization
<div style="display: flex; justify-content: center; align-items: flex-start; margin-top: 20px;">
<img src="./media/modular-code/testing_module.png" alt="testing a single module" width="300" style="margin-right: 20px;">
<div>
<ul>
<li>poor readability</li>
<li class="fragment" data-fragment-index="1">repetition</li>
</ul>
</div>
</div>
===
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## Repetitive code
- Don't Repeat Yourself (DRY): place reused code into a function
- Identify functions units by their _action_
- (e.g. "plot", "transform", "extract", "save")
<div style="text-align: center; margin-top: 20px;">
<img alt="DRY" src="./media/modular-code/dry-vs-wet-code.jpg" style="max-width: 100%; height: auto; max-height: 300px;">
</div>
===
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## Identifying opportunities for modularization
<div style="display: flex; justify-content: center; align-items: flex-start; margin-top: 20px;">
<img src="./media/modular-code/testing_module.png" alt="testing a single module" width="300" style="margin-right: 20px;">
<div>
<ul>
<li>poor readability</li>
<li>repetition</li>
<li class="fragment" data-fragment-index="1">nested code</li>
</ul>
</div>
</div>
===
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## Target nested code
Nested code is a prime target for modularization:
```python
def sum_even_numbers(bottom, top):
if top > bottom:
sum = 0
for x in range(bottom, top):
if x % 2 == 0:
sum += x
return sum
return 0
sum_even_numbers(2, 14) # 42
```
===
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## Reduce nestedness...
... by extracting modules:
```python
def _filter_even(number):
if number % 2 == 0:
return number
return 0
def sum_even_numbers(bottom, top):
if top < bottom:
return 0
sum = 0
for x in range(bottom, top):
sum += _filter_even(x)
return sum
sum_even_numbers (2,14) # 42
```
===
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## Identifying opportunities for modularization
<div style="display: flex; justify-content: center; align-items: flex-start; margin-top: 20px;">
<img src="./media/modular-code/testing_module.png" alt="testing a single module" width="300" style="margin-right: 20px;">
<div>
<ul>
<li>poor readability</li>
<li>repetition</li>
<li>nested code</li>
</ul>
</div>
</div>
===
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## Let tests help you
- Write tests for each individual module
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- Use the test-writing procedure to look critically at the module's function:
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- Is the input/output clear?
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- What can you not yet test? Extract it into a new module.
