Do you want simpler Python code? You always start a project with the best intentions, a clean codebase, and a nice structure. But over time, there are changes to your apps, and things can get a little messy.

If you can write and maintain clean, simple Python code, then it’ll save you lots of time in the long term. You can spend less time testing, finding bugs, and making changes when your code is well laid out and simple to follow.

In this tutorial you’ll learn:

  • How to measure the complexity of Python code and your applications
  • How to change your code without breaking it
  • What the common issues in Python code that cause extra complexity are and how you can fix them

Throughout this tutorial, I’m going to use the theme of subterranean railway networks to explain complexity because navigating a subway system in a large city can be complicated! Some are well designed, and others seem overly complex.

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Code Complexity in Python

The complexity of an application and its codebase is relative to the task it’s performing. If you’re writing code for NASA’s jet propulsion laboratory (literally rocket science), then it’s going to be complicated.

The question isn’t so much, “Is my code complicated?” as, “Is my code more complicated than it needs to be?”

The Tokyo railway network is one of the most extensive and complicated in the world. This is partly because Tokyo is a metropolis of over 30 million people, but it’s also because there are 3 networks overlapping each other.

Anthony Shaw on the Tokyo Metro
The author of this article getting lost on the Tokyo Metro

There are the Toei and Tokyo Metro rapid-transport networks as well as the Japan Rail East trains going through Central Tokyo. To even the most experienced traveler, navigating central Tokyo can be mind-bogglingly complicated.

Here is a map of the Tokyo railway network to give you some perspective:

Map of the Tokyo railway network
Image: Tokyo Metro Co.

If your code is starting to look a bit like this map, then this is the tutorial for you.

First, we’ll go through 4 metrics of complexity that can give you a scale to measure your relative progress in the mission to make your code simpler:

Code Complexity Metrics Table of Contents

After you’ve explored the metrics, you’ll learn about a tool called wily to automate calculating those metrics.

Metrics for Measuring Complexity

Much time and research have been put into analyzing the complexity of computer software. Overly complex and unmaintainable applications can have a very real cost.

The complexity of software correlates to the quality. Code that is easy to read and understand is more likely to be updated by developers in the future.

Here are some metrics for programming languages. They apply to many languages, not just Python.

Lines of Code

LOC, or Lines of Code, is the crudest measure of complexity. It is debatable whether there is any direct correlation between the lines of code and the complexity of an application, but the indirect correlation is clear. After all, a program with 5 lines is likely simpler than one with 5 million.

When looking at Python metrics, we try to ignore blank lines and lines containing comments.

Lines of code can be calculated using the wc command on Linux and Mac OS, where is the name of the file you want to measure:

If you want to add the combined lines in a folder by recursively searching for all .py files, you can combine wc with the find command:

$ find . -name \*.py | xargs wc -l

For Windows, PowerShell offers a word count command in Measure-Object and a recursive file search in Get-ChildItem:

$ Get-ChildItem -Path *.py -Recurse | Measure-Object Line 

In the response, you will see the total number of lines.

Why are lines of code used to quantify the amount of code in your application? The assumption is that a line of code roughly equates to a statement. Lines is a better measure than characters, which would include whitespace.

In Python, we are encouraged to put a single statement on each line. This example is 9 lines of code:

 1 x = 5
 2 value = input("Enter a number: ")
 3 y = int(value)
 4 if x < y:
 5  print(f"{x} is less than {y}")
 6 elif x == y:
 7  print(f"{x} is equal to {y}")
 8 else:
 9  print(f"{x} is more than {y}")

If you used only lines of code as your measure of complexity, it could encourage the wrong behaviors.

Python code should be easy to read and understand. Taking that last example, you could reduce the number of lines of code to 3:

 1 x = 5; y = int(input("Enter a number:"))
 2 equality = "is equal to" if x == y else "is less than" if x < y else "is more than"
 3 print(f"{x} {equality} {y}")

But the result is hard to read, and PEP 8 has guidelines around maximum line length and line breaking. You can check out How to Write Beautiful Python Code With PEP 8 for more on PEP 8.

This code block uses 2 Python language features to make the code shorter:

  • Compound statements: using ;
  • Chained conditional or ternary statements: name = value if condition else value if condition2 else value2

We have reduced the number of lines of code but violated one of the fundamental laws of Python:

“Readability counts”

— Tim Peters, Zen of Python

This shortened code is potentially harder to maintain because code maintainers are humans, and this short code is harder to read. We will explore some more advanced and useful metrics for complexity.

Cyclomatic Complexity

Cyclomatic complexity is the measure of how many independent code paths there are through your application. A path is a sequence of statements that the interpreter can follow to get to the end of the application.

One way to think of cyclomatic complexity and code paths is imagine your code is like a railway network.

For a journey, you may need to change trains to reach your destination. The Lisbon Metropolitan railway system in Portugal is simple and easy to navigate. The cyclomatic complexity for any trip is equal to the number of lines you need to travel on:

Map to Lisbon Subway map
Image: Metro Lisboa

If you needed to get from Alvalade to Anjos, then you would travel 5 stops on the linha verde (green line):

Map to Lisbon Subway map
Image: Metro Lisboa

This trip has a cyclomatic complexity of 1 because you only take 1 train. It’s an easy trip. That train is equivalent in this analogy to a code branch.

If you needed to travel from the Aeroporto (airport) to sample the food in the district of Belém, then it’s a more complicated journey. You would have to change trains at Alameda and Cais do Sodré:

Map to Lisbon Subway map
Image: Metro Lisboa

This trip has a cyclomatic complexity of 3, because you take 3 trains. You might be better off taking a taxi!

Seeing as how you’re not navigating Lisbon, but rather writing code, the changes of train line become a branch in execution, like an if statement. Let’s explore this example:

There is only 1 way this code can be executed, so it has a cyclomatic complexity of 1.

If we add a decision, or branch to the code as an if statement, it increases the complexity:

Even though there is only 1 way this code can be executed, as x is a constant, this has a cyclomatic complexity of 2. All of the cyclomatic complexity analyzers will treat an if statement as a branch.

This is also an example of overly complex code. The if statement is useless as x has a fixed value. You could simply refactor this example to the following:

That was a toy example, so let’s explore something a little more real.

main() has a cyclomatic complexity of 5. I’ll comment each branch in the code so you can see where they are:

import sys def main():
 if len(sys.argv) > 1: # 1
 filepath = sys.argv[1] else: print("Provide a file path") exit(1)
 if filepath: # 2
 with open(filepath) as fp: # 3
 for line in fp.readlines(): # 4
 if line != "\n": # 5
 print(line, end="") if __name__ == "__main__": # Ignored. main()

There are certainly ways that code can be refactored into a far simpler alternative. We’ll get to that later.

In the following examples, we will use the radon library from PyPi to calculate metrics. You can install it now:

To calculate cyclomatic complexity using radon, you can save the example into a file called and use radon from the command line.

The radon command takes 2 main arguments:

  1. The type of analysis (cc for cyclomatic complexity)
  2. A path to the file or folder to analyze

Execute the radon command with the cc analysis against the file. Adding -s will give the cyclomatic complexity in the output:

$ radon cc -s
 F 4:0 main - B (6)

The output is a little cryptic. Here is what each part means:

  • F means function, M means method, and C means class.
  • main is the name of the function.
  • 4 is the line the function starts on.
  • B is the rating from A to F. A is the best grade, meaning the least complexity.
  • The number in parentheses, 6, is the cyclomatic complexity of the code.

Halstead Metrics

The Halstead complexity metrics relate to the size of a program’s codebase. They were developed by Maurice H. Halstead in 1977. There are 4 measures in the Halstead equations:

  • Operands are values and names of variables.
  • Operators are all of the built-in keywords, like if, else, for or while.
  • Length (N) is the number of operators plus the number of operands in your program.
  • Vocabulary (h) is the number of unique operators plus the number of unique operands in your a program.

There are then 3 additional metrics with those measures:

  • Volume (V) represents a product of the length and the vocabulary.
  • Difficulty (D) represents a product of half the unique operands and the reuse of operands.
  • Effort (E) is the overall metric that is a product of volume and difficulty.

All of this is very abstract, so let’s put it in relative terms:

  • The effort of your application is highest if you use a lot of operators and unique operands.
  • The effort of your application is lower if you use a few operators and fewer variables.

For the example, operators and operands both occur on the first line:

import sys # import (operator), sys (operand)

import is an operator, and sys is the name of the module, so it’s an operand.

In a slightly more complex example, there are a number of operators and operands:

if len(sys.argv) > 1: ...

There are 5 operators in this example:

Furthermore, there are 2 operands:

Be aware that radon only counts a subset of operators. For example, parentheses are excluded in any calculations.

To calculate the Halstead measures in radon, you can run the following command:

$ radon hal
 h1: 3
 h2: 6
 N1: 3
 N2: 6
 vocabulary: 9
 length: 9
 calculated_length: 20.264662506490406
 volume: 28.529325012980813
 difficulty: 1.5
 effort: 42.793987519471216
 time: 2.377443751081734
 bugs: 0.009509775004326938

Why does radon give a metric for time and bugs?

Halstead theorized that you could estimate the time taken in seconds to code by dividing the effort (E) by 18.

Halstead also stated that the expected number of bugs could be estimated dividing the volume (V) by 3000. Keep in mind this was written in 1977, before Python was even invented! So don’t panic and start looking for bugs just yet.

Maintainability Index

The maintainability index brings the McCabe Cyclomatic Complexity and the Halstead Volume measures in a scale roughly between zero and one-hundred.

If you’re interested, the original equation is as follows:

MI Equation

In the equation, V is the Halstead volume metric, C is the cyclomatic complexity, and L is the number of lines of code.

If you’re as baffled as I was when I first saw this equation, here’s it means: it calculates a scale that includes the number of variables, operations, decision paths, and lines of code.

It is used across many tools and languages, so it’s one of the more standard metrics. However, there are numerous revisions of the equation, so the exact number shouldn’t be taken as fact. radon, wily, and Visual Studio cap the number between 0 and 100.

On the maintainability index scale, all you need to be paying attention to is when your code is getting significantly lower (toward 0). The scale considers anything lower than 25 as hard to maintain, and anything over 75 as easy to maintain. The Maintainability Index is also referred to as MI.

The maintainability index can be used as a measure to get the current maintainability of your application and see if you’re making progress as you refactor it.

To calculate the maintainability index from radon, run the following command:

$ radon mi -s - A (87.42)

In this result, A is the grade that radon has applied to the number 87.42 on a scale. On this scale, A is most maintainable and F the least.

Using wily to Capture and Track Your Projects’ Complexity

wily is an open-source software project for collecting code-complexity metrics, including the ones we’ve covered so far like Halstead, Cyclomatic, and LOC. wily integrates with Git and can automate the collection of metrics across Git branches and revisions.

The purpose of wily is to give you the ability to see trends and changes in the complexity of your code over time. If you were trying to fine-tune a car or improve your fitness, you’d start off with measuring a baseline and tracking improvements over time.

Installing wily

wily is available on PyPi and can be installed using pip:

Once wily is installed, you have some commands available in your command-line:

  • wily build: iterate through the Git history and analyze the metrics for each file
  • wily report: see the historical trend in metrics for a given file or folder
  • wily graph: graph a set of metrics in an HTML file

Building a Cache

Before you can use wily, you need to analyze your project. This is done using the wily build command.

For this section of the tutorial, we will analyze the very popular requests package, used for talking to HTTP APIs. Because this project is open-source and available on GitHub, we can easily access and download a copy of the source code:

$ git clone
$ cd requests
$ ls
Pipfile.lock _appveyor docs pytest.ini
setup.cfg tests Pipfile appveyor.yml
ext requests tox.ini

Note: Windows users should use the PowerShell command prompt for the following examples instead of traditional MS-DOS Command-Line. To start the PowerShell CLI press Win+R and type powershell then Enter.

You will see a number of folders here, for tests, documentation, and configuration. We’re only interested in the source code for the requests Python package, which is in a folder called requests.

Call the wily build command from the cloned source code and provide the name of the source code folder as the first argument:

This will take a few minutes to analyze, depending on how much CPU power your computer has:

Screenshot capture of Wily build command

Collecting Data on Your Project

Once you have analyzed the requests source code, you can query any file or folder to see key metrics. Earlier in the tutorial, we discussed the following:

  • Lines of Code
  • Maintainability Index
  • Cyclomatic Complexity

Those are the 3 default metrics in wily. To see those metrics for a specific file (such as requests/, run the following command:

$ wily report requests/

wily will print a tabular report on the default metrics for each Git commit in reverse date order. You will see the most recent commit at the top and the oldest at the bottom:

Revision Author Date MI Lines of Code Cyclomatic Complexity
f37daf2 Nate Prewitt 2019-01-13 100 (0.0) 158 (0) 9 (0)
6dd410f Ofek Lev 2019-01-13 100 (0.0) 158 (0) 9 (0)
5c1f72e Nate Prewitt 2018-12-14 100 (0.0) 158 (0) 9 (0)
c4d7680 Matthieu Moy 2018-12-14 100 (0.0) 158 (0) 9 (0)
c452e3b Nate Prewitt 2018-12-11 100 (0.0) 158 (0) 9 (0)
5a1e738 Nate Prewitt 2018-12-10 100 (0.0) 158 (0) 9 (0)

This tells us that the requests/ file has:

  • 158 lines of code
  • A perfect maintainability index of 100
  • A cyclomatic complexity of 9

To see other metrics, you first need to know the names of them. You can see this by running the following command:

You will see a list of operators, modules that analyze the code, and the metrics they provide.

To query alternative metrics on the report command, add their names after the filename. You can add as many metrics as you wish. Here’s an example with the Maintainability Rank and the Source Lines of Code:

$ wily report requests/ maintainability.rank raw.sloc

You will see the table now has 2 different columns with the alternative metrics.

Graphing Metrics

Now that you know the names of the metrics and how to query them on the command line, you can also visualize them in graphs. wily supports HTML and interactive charts with a similar interface as the report command:

$ wily graph requests/ maintainability.mi

Your default browser will open with an interactive chart like this:

Screenshot capture of Wily graph command

You can hover over specific data points, and it will show the Git commit message as well as the data.

If you want to save the HTML file in a folder or repository, you can add the -o flag with the path to a file:

$ wily graph requests/ maintainability.mi -o my_report.html

There will now be a file called my_report.html that you can share with others. This command is ideal for team dashboards.

wily as a pre-commit Hook

wily can be configured so that before you commit changes to your project, it can alert you to improvements or degradations in complexity.

wily has a wily diff command, that compares the last indexed data with the current working copy of a file.

To run a wily diff command, provide the names of the files you have changed. For example, if I made some changes to requests/ you will see the impact on the metrics by running wily diff with the file path:

$ wily diff requests/ 

In the response, you will see all of the changed metrics, as well as the functions or classes that have changed for cyclomatic complexity:

Screenshot of the wily diff command

The diff command can be paired with a tool called pre-commit. pre-commit inserts a hook into your Git configuration that calls a script every time you run the git commit command.

To install pre-commit, you can install from PyPI:

Add the following to a .pre-commit-config.yaml in your projects root directory:

- repo: local hooks: - id: wily name: wily entry: wily diff verbose: true language: python additional_dependencies: [wily]

Once setting this, you run the pre-commit install command to finalize things:

Whenever you run the git commit command, it will call wily diff along with the list of files you’ve added to your staged changes.

wily is a useful utility to baseline the complexity of your code and measure the improvements you make when you start to refactor.

Refactoring in Python

Refactoring is the technique of changing an application (either the code or the architecture) so that it behaves the same way on the outside, but internally has improved. These improvements can be stability, performance, or reduction in complexity.

One of the world’s oldest underground railways, the London Underground, started in 1863 with the opening of the Metropolitan line. It had gas-lit wooden carriages hauled by steam locomotives. On the opening of the railway, it was fit for purpose. 1900 brought the invention of the electric railways.

By 1908, the London Underground had expanded to 8 railways. During the Second World War, the London Underground stations were closed to trains and used as air-raid shelters. The modern London Underground carries millions of passengers a day with over 270 stations:

London Underground in 1908
Joint London Underground Railways Map, c. 1908 (Image: Wikipedia)

It’s almost impossible to write perfect code the first time, and requirements change frequently. If you would have asked the original designers of the railway to design a network fit for 10 million passengers a day in 2020, they would not design the network that exists today.

Instead, the railway has undergone a series of continuous changes to optimize its operation, design, and layout to match the changes in the city. It has been refactored.

In this section, you’ll explore how to safely refactor by leveraging tests and tools. You’ll also see how to use the refactoring functionality in Visual Studio Code and PyCharm:

Refactoring Section Table of Contents

If the point of refactoring is to improve the internals of an application without impacting the externals, how do you ensure the externals haven’t changed?

Before you charge into a major refactoring project, you need to make sure you have a solid test suite for your application. Ideally, that test suite should be mostly automated, so that as you make changes, you see the impact on the user and address it quickly.

If you want to learn more about testing in Python, Getting Started With Testing in Python is a great place to start.

There is no perfect number of tests to have on your application. But, the more robust and thorough the test suite, the more aggressively you can refactor your code.

The two most common tasks you will perform when doing refactoring are:

  • Renaming modules, functions, classes, and methods
  • Finding usages of functions, classes, and methods to see where they are called

You can simply do this by hand using search and replace, but it is both time consuming and risky. Instead, there are some great tools to perform these tasks.

Using rope for Refactoring

rope is a free Python utility for refactoring Python code. It comes with an extensive set of APIs for refactoring and renaming components in your Python codebase.

rope can be used in two ways:

To use rope as a library, first install rope by executing pip:

It is useful to work with rope on the REPL so that you can explore the project and see changes in real time. To start, import the Project type and instantiate it with the path to the project:

>>> from rope.base.project import Project >>> proj = Project('requests')

The proj variable can now perform a series of commands, like get_files and get_file, to get a specific file. Get the file and assign it to a variable called api:

>>> [ for f in proj.get_files()]
['', '', ...,'', ''] >>> api = proj.get_file('')

If you wanted to rename this file, you could simply rename it on the filesystem. However, any other Python files in your project that imported the old name would now be broken. Let’s rename the to

>>> from rope.refactor.rename import Rename >>> change = Rename(proj, api).get_changes('new_api') >>>

Running git status, you will see that rope made some changes to the repository:

$ git status
On branch master
Your branch is up to date with 'origin/master'. Changes not staged for commit:
 (use "git checkout -- <file>..." to discard changes in working directory)  modified: requests/
 deleted: requests/ Untracked files:
 (use "git add <file>..." to include in what will be committed)  requests/.ropeproject/

The three changes made by rope are the following:

  1. Deleted requests/ and created requests/
  2. Modified requests/ to import from new_api instead of api
  3. Created a project folder named .ropeproject

To reset the change, run git reset.

There are hundreds of other refactorings that can be done with rope.

Using Visual Studio Code for Refactoring

Visual Studio Code opens up a small subset of the refactoring commands available in rope through its own UI.

You can:

  1. Extract variables from a statement
  2. Extract methods from a block of code
  3. Sort imports into a logical order

Here is an example of using the Extract methods command from the command palette:

Screenshot of Visual Studio Code refactoring

Using PyCharm for Refactoring

If you use or are considering using PyCharm as a Python editor, it’s worth taking note of the powerful refactoring capabilities it has.

You can access all the refactoring shortcuts with the Ctrl+T command on Windows and macOS. The shortcut to access refactoring in Linux is Ctrl+Shift+Alt+T.

Finding Callers and Usages of Functions and Classes

Before you remove a method or class or change the way it behaves, you’ll need to know what code depends on it. PyCharm can search for all usages of a method, function, or class within your project.

To access this feature, select a method, class, or variable by right-clicking and select Find Usages:

Finding usages in PyCharm

All of the code that uses your search criteria is shown in a panel at the bottom. You can double-click on any item to navigate directly to the line in question.

Some of the other refactoring commands include the ability to:

  • Extract methods, variables, and constants from existing code
  • Extract abstract classes from existing class signatures, including the ability to specify abstract methods
  • Rename practically anything, from a variable to a method, file, class, or module

Here is an example of renaming the same module you renamed earlier using the rope module to

How to rename methods in pycharm

The rename command is contextualized to the UI, which makes refactoring quick and simple. It has updated the imports automatically in with the new module name.

Another useful refactor is the Change Signature command. This can be used to add, remove, or rename arguments to a function or method. It will search for usages and update them for you:

Changing method signatures in PyCharm

You can set default values and also decide how the refactoring should handle the new arguments.

Refactoring is an important skill for any developer. As you’ve learned in this chapter, you aren’t alone. The tools and IDEs already come with powerful refactoring features to be able to make changes quickly.

Complexity Anti-Patterns

Now that you know how complexity can be measured, how to measure it, and how to refactor your code, it’s time to learn 5 common anti-patterns that make code more complex than it need be:

Anti-Patterns Table of Contents

If you can master these patterns and know how to refactor them, you’ll soon be on track (pun intended) to a more maintainable Python application.

1. Functions That Should Be Objects

Python supports procedural programming using functions and also inheritable classes. Both are very powerful and should be applied to different problems.

Take this example of a module for working with images. The logic in the functions has been removed for brevity:

# def load_image(path): with open(path, "rb") as file: fb = file.load() image = img_lib.parse(fb) return image def crop_image(image, width, height): ... return image def get_image_thumbnail(image, resolution=100): ... return image

There are a few issues with this design:

  1. It’s not clear if crop_image() and get_image_thumbnail() modify the original image variable or create new images. If you wanted to load an image then create both a cropped and thumbnail image, would you have to copy the instance first? You could read the source code in the functions, but you can’t rely on every developer doing this.

  2. You have to pass the image variable as an argument in every call to the image functions.

This is how the calling code might look:

from imagelib import load_image, crop_image, get_image_thumbnail image = load_image('~/face.jpg')
image = crop_image(image, 400, 500)
thumb = get_image_thumbnail(image)

Here are some symptoms of code using functions that could be refactored into classes:

  • Similar arguments across functions
  • Higher number of Halstead h2 unique operands
  • Mix of mutable and immutable functions
  • Functions spread across multiple Python files

Here is a refactored version of those 3 functions, where the following happens:

  • .__init__() replaces load_image().
  • crop() becomes a class method.
  • get_image_thumbnail() becomes a property.

The thumbnail resolution has become a class property, so it can be changed globally or on that particular instance:

# class Image(object): thumbnail_resolution = 100 def __init__(self, path): ... def crop(self, width, height): ... @property def thumbnail(self): ... return thumb

If there were many more image-related functions in this code, the refactoring to a class could make a drastic change. The next consideration would be the complexity of the consuming code.

This is how the refactored example would look:

from imagelib import Image image = Image('~/face.jpg')
image.crop(400, 500)
thumb = image.thumbnail

In the resulting code, we have solved the original problems:

  • It is clear that thumbnail returns a thumbnail since it is a property, and that it doesn’t modify the instance.
  • The code no longer requires creating new variables for the crop operation.

2. Objects That Should Be Functions

Sometimes, the reverse is true. There is object-oriented code which would be better suited to a simple function or two.

Here are some tell-tale signs of incorrect use of classes:

  • Classes with 1 method (other than .__init__())
  • Classes that contain only static methods

Take this example of an authentication class:

# class Authenticator(object): def __init__(self, username, password): self.username = username self.password = password def authenticate(self): ... return result

It would make more sense to just have a simple function named authenticate() that takes username and password as arguments:

# def authenticate(username, password): ... return result

You don’t have to sit down and look for classes that match these criteria by hand: pylint comes with a rule that classes should have a minimum of 2 public methods. For more on PyLint and other code quality tools, you can check out Python Code Quality.

To install pylint, run the following command in your console:

pylint takes a number of optional arguments and then the path to one or more files and folders. If you run pylint with its default settings, it’s going to give a lot of output as pylint has a huge number of rules. Instead, you can run specific rules. The too-few-public-methods rule id is R0903. You can look this up on the documentation website:

$ pylint --disable=all --enable=R0903 requests
************* Module requests.auth
requests/ R0903: Too few public methods (1/2) (too-few-public-methods)
************* Module requests.models
requests/ R0903: Too few public methods (1/2) (too-few-public-methods) -----------------------------------
Your code has been rated at 9.99/10

This output tells us that contains 2 classes that have only 1 public method. Those classes are on lines 72 and 100. There is also a class on line 60 of with only 1 public method.

3. Converting “Triangular” Code to Flat Code

If you were to zoom out on your source code and tilt your head 90 degrees to the right, does the whitespace look flat like Holland or mountainous like the Himalayas? Mountainous code is a sign that your code contains a lot of nesting.

Here’s one of the principles in the Zen of Python:

“Flat is better than nested”

— Tim Peters, Zen of Python

Why would flat code be better than nested code? Because nested code makes it harder to read and understand what is happening. The reader has to understand and memorize the conditions as they go through the branches.

These are the symptoms of highly nested code:

  • A high cyclomatic complexity because of the number of code branches
  • A low Maintainability Index because of the high cyclomatic complexity relative to the number of lines of code

Take this example that looks at the argument data for strings that match the word error. It first checks if the data argument is a list. Then, it iterates over each and checks if the item is a string. If it is a string and the value is "error", then it returns True. Otherwise, it returns False:

def contains_errors(data): if isinstance(data, list): for item in data: if isinstance(item, str): if item == "error": return True return False

This function would have a low maintainability index because it is small, but it has a high cyclomatic complexity.

Instead, we can refactor this function by “returning early” to remove a level of nesting and returning False if the value of data is not list. Then using .count() on the list object to count for instances of "error". The return value is then an evaluation that the .count() is greater than zero:

def contains_errors(data): if not isinstance(data, list): return False return data.count("error") > 0

Another technique for reducing nesting is to leverage list comprehensions. This common pattern of creating a new list, going through each item in a list to see if it matches a criterion, then adding all matches to the new list:

results = []
for item in iterable: if item == match: results.append(item)

This code can be replaced with a faster and more efficient list comprehension.

Refactor the last example into a list comprehension and an if statement:

results = [item for item in iterable if item == match]

This new example is smaller, has less complexity, and is more performant.

If your data is not a single dimension list, then you can leverage the itertools package in the standard library, which contains functions for creating iterators from data structures. You can use it for chaining iterables together, mapping structures, cycling or repeating over existing iterables.

Itertools also contains functions for filtering data, like filterfalse(). For more on Itertools, check out Itertools in Python 3, By Example.

One of Python’s most powerful and widely used core types is the dictionary. It’s fast, efficient, scalable, and highly flexible.

If you’re new to dictionaries, or think you could leverage them more, you can read Dictionaries in Python for more information.

It does have one major side-effect: when dictionaries are highly nested, the code that queries them becomes nested too.

Take this example piece of data, a sample of the Tokyo Metro lines you saw earlier:

data = { "network": { "lines": [ { "name.en": "Ginza", "": "銀座線", "color": "orange", "number": 3, "sign": "G" }, { "name.en": "Marunouchi", "": "丸ノ内線", "color": "red", "number": 4, "sign": "M" } ] }

If you wanted to get the line that matched a certain number, this could be achieved in a small function:

def find_line_by_number(data, number): matches = [line for line in data if line['number'] == number] if len(matches) > 0: return matches[0] else: raise ValueError(f"Line {number} does not exist.")

Even though the function itself is small, calling the function is unnecessarily complicated because the data is so nested:

>>> find_line_by_number(data["network"]["lines"], 3)

There are third party tools for querying dictionaries in Python. Some of the most popular are JMESPath, glom, asq, and flupy.

JMESPath can help with our train network. JMESPath is a querying language designed for JSON, with a plugin available for Python that works with Python dictionaries. To install JMESPath, do the following:

Then open up a Python REPL to explore the JMESPath API, copying in the data dictionary. To get started, import jmespath and call search() with a query string as the first argument and the data as the second. The query string "network.lines" means return data['network']['lines']:

>>> import jmespath >>>"network.lines", data)
[{'name.en': 'Ginza', '': '銀座線',
 'color': 'orange', 'number': 3, 'sign': 'G'}, 
 {'name.en': 'Marunouchi', '': '丸ノ内線',
 'color': 'red', 'number': 4, 'sign': 'M'}]

When working with lists, you can use square brackets and provide a query inside. The “everything” query is simply *. You can then add the name of the attribute inside each matching item to return. If you wanted to get the line number for every line, you could do this:

>>>"network.lines[*].number", data)
[3, 4]

You can provide more complex queries, like a == or <. The syntax is a little unusual for Python developers, so keep the documentation handy for reference.

If we wanted to find the line with the number 3, this can be done in a single query:

>>>"network.lines[?number==`3`]", data)
[{'name.en': 'Ginza', '': '銀座線', 'color': 'orange', 'number': 3, 'sign': 'G'}]

If we wanted to get the color of that line, you could add the attribute in the end of the query:

>>>"network.lines[?number==`3`].color", data)

JMESPath can be used to reduce and simplify code that queries and searches through complex dictionaries.

5. Using attrs and dataclasses to Reduce Code

Another goal when refactoring is to simply reduce the amount of code in the codebase while achieving the same behaviors. The techniques shown so far can go a long way to refactoring code into smaller and simpler modules.

Some other techniques require a knowledge of the standard library and some third party libraries.

What Is Boilerplate?

Boilerplate code is code that has to be used in many places with little or no alterations.

Taking our train network as an example, if we were to convert that into types using Python classes and Python 3 type hints, it might look something like this:

from typing import List class Line(object): def __init__(self, name_en: str, name_jp: str, color: str, number: int, sign: str): self.name_en = name_en self.name_jp = name_jp self.color = color self.number = number self.sign = sign def __repr__(self): return f"<Line {self.name_en} color='{self.color}' number={self.number} sign='{self.sign}'>" def __str__(self): return f"The {self.name_en} line" class Network(object): def __init__(self, lines: List[Line]): self._lines = lines @property def lines(self) -> List[Line]: return self._lines

Now, you might also want to add other magic methods, like .__eq__(). This code is boilerplate. There’s no business logic or any other functionality here: we’re just copying data from one place to another.

A Case for dataclasses

Introduced into the standard library in Python 3.7, with a backport package for Python 3.6 on PyPI, the dataclasses module can help remove a lot of boilerplate for these types of classes where you’re just storing data.

To convert the Line class above to a dataclass, convert all of the fields to class attributes and ensure they have type annotations:

from dataclasses import dataclass @dataclass
class Line(object): name_en: str name_jp: str color: str number: int sign: str

You can then create an instance of the Line type with the same arguments as before, with the same fields, and even .__str__(), .__repr__(), and .__eq__() are implemented:

>>> line = Line('Marunouchi', "丸ノ内線", "red", 4, "M") >>> line.color
red >>> line2 = Line('Marunouchi', "丸ノ内線", "red", 4, "M") >>> line == line2

Dataclasses are a great way to reduce code with a single import that’s already available in the standard library. For a full walkthrough, you can checkout The Ultimate Guide to Data Classes in Python 3.7.

Some attrs Use Cases

attrs is a third party package that’s been around a lot longer than dataclasses. attrs has a lot more functionality, and it’s available on Python 2.7 and 3.4+.

If you are using Python 3.5 or below, attrs is a great alternative to dataclasses. Also, it provides many more features.

The equivalent dataclasses example in attrs would look similar. Instead of using type annotations, the class attributes are assigned with a value from attrib(). This can take additional arguments, such as default values and callbacks for validating input:

from attr import attrs, attrib @attrs
class Line(object): name_en = attrib() name_jp = attrib() color = attrib() number = attrib() sign = attrib()

attrs can be a useful package for removing boilerplate code and input validation on data classes.


Now that you’ve learned how to identify and tackle complicated code, think back to the steps you can now take to make your application easier to change and manage:

  • Start off by creating a baseline of your project using a tool like wily.
  • Look at some of the metrics and start with the module that has the lowest maintainability index.
  • Refactor that module using the safety provided in tests and the knowledge of tools like PyCharm and rope.

Once you follow these steps and the best practices in this article, you can do other exciting things to your application, like adding new features and improving performance.