Structuring code

Last updated on 2026-03-13 | Edit this page

Estimated time: 25 minutes

Overview

Questions

  • How can we create simpler and more modular codes?

Objectives

  • Explain the expression “separation of concerns”
  • Explain the expression “levels of abstractions”
  • Explain the expression “single responsibility principle”
  • Explain the expression “dataflow”
  • Analyze an algorithm for levels of abstractions, separable concerns and dataflow
  • Create focussed, modular algorithms

Structuring your code

Discussion

Nutella Cake: What’s wrong in here?

Pour the preparation onto the dough. Wait for it to cool. Once it is cool, sprinkle with icing sugar.

The dough is composed of 250g of flour, 125g of butter and one egg yolk. It should be blind-baked at 180°C until golden brown.

The preparation consists of 200g of Nutella, 3 large spoonfuls of crème fraîche, 2 egg yolks and 20g of butter.

Add the Nutella and the butter to a pot where you have previously mixed the crème fraîche, the egg yolk and the vanilla extract. Cook and mix on low heat until homogeneous.

The dough, a pâte sablée, is prepared by lightly mixing by hand a pinch of salt, 250g of flour and 125g of diced butter until the butter is mostly all absorbed. Then add the egg yolk and 30g of cold water. The dough should look like coarse sand, thus its name.

Spread the dough into a baking sheet. If using it for a pie with a pre-cooked or raw filling, first cook the dough blind at 200°C until golden brown. If the dough and filling will be cooked together you can partially blind-cook the dough at 180°C for 15mn for added crispiness.

Enjoy! It’s now ready to serve!

Code is often as unreadable as this recipe, which makes things very hard to yourself and other people working on the code. Luckily, there are some rules and principles that can be followed to fix this and are a sign of good quality code.

In this lesson we will cover some of them, using the above recipe as an example, when relevant.

Separation of concerns

Separation of Concerns (SoC) is a fundamental design principle in software engineering that involves dividing a program into distinct sections, each addressing a separate concern. A concern is a specific aspect of functionality, behaviour, or responsibility within the system.

One of the first issues with the recipe above was that all the different elements were mixed up. The ingredients and the different steps of the preparation are all described together, making it impossible to figure out where to look to find out what to buy or how to prepare the dough or the filling.

Like the recipe, a piece of code should be split into multiple parts, each of them dealing with one aspect of the workflow. This makes the code more readable, and also more reusable, since the different parts could be mixed if their functionality were clearly separated.

For example, the following code will have clear separation of concerns along the whole workflow:

PYTHON 

def main(filename, value):
  # reading input files is one thing
  data = read_input(filename)

  # creating complex objects is another
  simulator = Simulator(data)

  # compute is still something else
  result = simulator.compute(value)

  # Saving data is another
  save(result)
Challenge

What concerns can you identify in the recipe?

Using the recipe above, what concerns can you identify? Concerns might be nested!

The first obvious separation could be between Ingredients and Preparation. However, there is also another level between Dough and Filling, so a possible structure for the concerns could be:

  • Ingredients
    • Dough:
      • …
    • Filling:
      • …
  • Preparation
    • Dough:
      • …
    • Filling:
      • …
    • Finishing

But there is another, more reusable solution, to separate the Dough and the Filling first, and then the Ingredients and the Preparation. This way, the same Dough could be used in different cake recipes, and the same for the Filling.

  • Dough
    • Ingredients:
      • …
    • Preparation:
      • …
  • Filling
    • Ingredients:
      • …
    • Preparation:
      • …
    • Finishing:
    • Combine the dough and the filling prepared as above and…

Sometimes, there are multiple ways of separating the concerns, so spend some time upfront thinking on the simplest, yet more reusable solution for your code.

Levels of abstraction

Abstraction is a concept in computer science and software engineering that involves generalizing concrete details to focus on more important, generic aspects. It helps in managing complexity by hiding implementation details and exposing only the necessary parts.

Let’s consider the following code:

PYTHON 

def analyse(input_path):
    data = load_data(input_path)

    # special fix for bad data point
    data[0].x["january"] = 0

    results = process_data(data)
    return results

The “special fix” mixes levels of abstraction. Our top level function now depends on the structure of data. If the way data is stored changes (a low level detail) we now also need to change analyse which is a high level function directing the flow of the program.

The following code encapsulates those low level changes in another function, honouring the different levels of abstraction.

PYTHON 

def analyse(input_path):
    data = load_data(input_path)

    cleaned_data = clean_data(data)

    results = process_data(cleaned_data)
    return results

Now our analyse function does not need to know anything about the inside workings of the data object.

Single responsibility principle

The Single Responsibility Principle (SRP) states that a class or module should have only one reason to change, meaning it should have only one responsibility or purpose. This principle helps in creating more maintainable, reusable, and flexible software designs.

This goes into a deeper level than the abstractions described above, and it is probably the one harder to do right, besides its apparent simplicity. It literally means that each function or method should do one thing only. This typically means also that is short, easier to read, to debug and to re-use.

Let’s consider the following code. It uses Pandas to load some data from either an Excel file or a CSV file. In the second case, it also manipulates some columns to make sure the right ones are there.

PYTHON 

def load_data(filename):
    if filename.suffix == ".xlsx":
        data = pd.read_excel(filename, usecols="A:B")

    elif filename.suffix == ".csv":
        data = pd.read_csv(filename)
        data["datetime"] = data["date"] + " " + data["time"]
        data = data.drop(columns=["date", "time"])

    else:
        raise RuntimeError("The file must be an Excel or a CSV file")

    assert data.columns == ["datetime", "value"]
    return data

This function is doing too many things despite being just 13 lines long, and might need to change for several, unrelated reasons. If we work on the separation of concerns, we will have several functions that:

  1. Select how to read the data based on the extension.
  2. Read an Excel file
  3. Read a CSV file
  4. Validate the data structure

PYTHON 

def load_data(filename):
    """Select how to load the data based on the file extension."""
    if filename.suffix == ".xlsx":
        data = load_excel(filename)
    elif filename.suffix == ".csv":
        data = load_csv(filename)
    else:
        raise RuntimeError("The file must be an Excel or a CSV file.")
    return data

def load_excel(filename):
    """Loads an Excel file, picking only the first 2 columns."""
    data = pd.read_excel(filename, usecols="A:B")
    validate(data)
    return data

def load_csv(filename):
    """Loads a CSV file, ensuring the date and time are combined."""
    data = pd.read_csv(filename)
    data["datetime"] = data["date"] + " " + data["time"]
    data = data.drop(columns=["date", "time"])
    validate(data)
    return data

def validate(data):
    """Checks that the data has the right structure."""
    assert data.columns == ["datetime", "value"]

The overall length of the code is much more, but now each function does a very specific thing, and only one thing, and well documented. The load_data function only needs to change if more formats are supported. The load_excel only if the structure of the Excel file changes and the same goes for the load_csv. And the validate data one, only changes if there are more or different things to validate. The code becomes cleaner, easier to read and to re-use.

Other considerations

Code legibility

Code is often meant to be read by an audience who has either not so much familiarity with it or has not work on it for a while. This audience is normally future-you reading past-you’s code.

For example, given a point with coordinates point[0], point[1], point[2], what do each of them mean? Does 0 corresponds to x, y, or z? Or is it r, theta or z? Or something else altogether?

Intelligible code aims to:

  • Use descriptive names for variables and functions. The times where names should fit within 8 characters, and whole lines be less than 72 are long past.
  • Avoid sources of confusion (What does index 0 reference?)
  • Lessen the cognitive load (Before using c don’t forget to call the function nothing_to_do_with_c because of historical implementation detail).

Some aspects of code legibility will be sorted - or at least flagged out - by the formatters and linters already discussed, but others do require a conscious effort by the developer.

Use of global variables

While global variables were common in the past, they have become less and less popular due to the problems they might bring to the code. Avoid the use global variables, when possible. They make the dataflow complex by essence.

Let’s consider the following code:

PYTHON 

SOME_GLOBAL_VARIABLE = 42

def compute_a(measurements):
    ...
    result *= SOME_GLOBAL_VARIABLE
    return result

Now the result of compute_a has a hidden dependency. It’s never clear whether calling it twice with the same input measurements will yield the same result, as SOME_GLOBAL_VARIABLE might have changed.

In general make sure that all variables have the most limited scope possible. If they’re only needed within a single function define them there. Wherever possible treat variables with wide scope as constants (or make them actual constants if your language supports it) so you know they’re not being modified anywhere.

Having said that, there are situations where global variables are needed, or are just unavoidable. A typical example would be accessing a database. That database is just one of such global variables, accessible anywhere in the program, and might change in different places. There is no way around that, but just make sure that measures are in place to ensure that the state of the database is known at any given time.

Mixing IO and creating objects or making calculations

Input and output (IO) operations are tricky because they depend on the state of the system - the input file(s) contents - and change the state of the system - creating new file(s). They need to be handled with care and, indeed, some pure functional languages like Haskell consider them dirty operations because of that.

The general rule is that IO operations should happen within their own functions or methods, and and just pass their contents around. This is a direct conclusion of the Separation of concerns principle described above, as applied to IO operations.

For example, objects that need files to be created are hard to create and re-create, especially during testing. Make the contents of the file an input for the object creation function, instead of the filename or file object. You might want to use factory methods to support this approach.

For example, don’t do this:

PYTHON 

class MyModel
    def __init__(self, some_array):
        self.some_array = array
        # BAD!! Now you need to carry this file around every time you want to
        # instantiate MyModel
        self.aaa = read_aaa("somefile.aaa")

Do this instead, so you can instantiate MyModel without having the file at hand:

PYTHON 

class MyModel
    @classmethod
    def from_file(cls, array, filename):
        aaa = read_aaa(filename)
        return cls(array, aaa)

    def __init__(self, some_array, aaa):
        self.some_array = array
        self.aaa = aaa

Dataflow

Often code is just a sequence of transformations on data, so if we have the following set of actions:

  1. data measurements is read from input
  2. data a and b are produced from measurements, independently
  3. data result is produced from a and b

The code should reflect that structure:

PYTHON 

def read_experiment(filename):
    ...
    return measurements

def compute_a(measurements):
    ...
    return a

def compute_b(measurements):
    ...
    return b

def compute_result(a, b):
    ...
    return result
  1. One function to read Experiment
  2. One function/class to compute a: It takes measurements as input and returns the result a
  3. One function/class to compute b: It takes measurements as input and returns the result b
  4. One function/class to compute Result: It takes a and b as input and returns the result result:

Unnecessary arguments and tangled dependencies

What sometimes happens is that there are unnecessary arguments and tangled dependencies that can make the code harder to understand and, indeed, be a source of errors.

Look at the following alternative code:

PYTHON 

# BAD! Unnecessary arguments. Now result depends on a, b, AND measurements
def compute_result(a, b, measurements):
    pass

Did we not say the result depends on a and b alone? The next programmer to look at the code (e.g. future you) won’t know that. Computing result is no longer separate from measurements.

Modifying an input argument

Some languages are designed so sometimes you don’t have any choice but to modify an input argument. Still, wherever possible avoid doing it because if you do not, other functions using that input argument will need to know who used it first!

If possible avoid doing this.

PYTHON 

def compute_a(measurements):
    measurements[1] *= 2
    ...

def compute_b(measurements):
    some_result = measurements[1] * 0.5
    ...

def compute(measurements):
    compute_a(measurements)
    compute_b(measurements)
    ...

If compute_a takes place before compute_b the result would be different than if it is the other way around. However, there is no clue in the compute function indicating that is the case.

If you do need to modify the inputs then provide them as outputs of the function, such that it is clear by successive functions that they are using modified versions of the inputs.

PYTHON 

def compute_a(measurements):
    measurements[1] *= 2
    ...
    return measurements

def compute_b(measurements):
    some_result = measurements[1] * 0.5
    ...
    return some_result

def compute(measurements):
    measurements = compute_a(measurements)
    some_result = compute_b(measurements)
    ...

Now it is clear that compute_b is using a measurements object that has been outputted by compute_a and, therefore, it should expect any changes made by it.

Key Points
  • Code is read more often than written
  • The structure of the code should follow a well written explanation of its algorithm
  • Separate level of abstractions (or details) in a problem must be reflected in the structure of the code, e.g. with separate functions
  • Separable concerns (e.g. reading an input file to create X, creating X, computing stuff with X, saving results to file) must be reflected in the structure of the code