Clean Contracts - a guide on smart contract patterns & practices

Blockchain and smart contract development require solid engineering practices, rigorous testing, and a strong security mindset. This guide covers several patterns, practices, and principles to reduce the risks associated with it.

Blockchain and smart contract development are still relatively new and highly experimental. They require a different engineering mindset than traditional web, or app development where ‘move fast and break things’ has become the norm.

Blockchain development is much more like hardware, or financial service development. Smart contracts are complex instruments that offer the possibility to have self-enforcing contracts including transparent, tamper-proof, and immutable information. They have the authority to allocate high-value resources between complex systems. Often working autonomously. With huge financial loss at risk. This makes smart contracts critical components in these systems. Development of such components requires more investment, design, and effort upfront. Solid engineering practices, rigorous testing, and a strong security mindset.

In a series of blog posts, I’m planning to go through several patterns, practices, and principles that can be applied to blockchain and smart contract development to reduce the risks associated with it.

Clean Code


In this first article, I’ll cover more general engineering practices based on the concepts of Clean Code. Clean code is a well-known concept within the software development industry. Robert C Martin, also known as 'Uncle Bob' wrote the famous handbook. Its principles go back to even the earlier concepts of the Agile manifesto and software craftsmanship. It’s a knowledge base for the way we think, write, and read code. Build on decades of wisdom in software development.

“Truth can only be found in one place: the code“ - Robert C. Martin

Clean code is code that is easy to read. Easy to understand. And easy to maintain. Clean code is an attempt to make sense of the complexity of systems that we’re dealing with. It’s a defense mechanism that can provide guidance when you’re not sure how a change would affect the codebase. While there’s valid criticism on the handbook and the examples are considered outdated, the principles are still very relevant. Especially for object-oriented languages, such as Solidity. They apply to designing and writing secure, open, and immutable code, such as smart contracts.



The naming of contracts, functions, or variables should reveal the intention, why it exists, and how it is used. If a name requires comments to explain, then it doesn’t reveal its intent.

Be consistent in your naming throughout your code. Especially when you’re working with a large codebase. Use the same names for abstract concepts, throughout multiple contracts.

Use pronounceable names.

Use searchable names.

"There are only two hard things in Computer Science: cache invalidation and naming things." - Phil Karlton


Structure & Ordering

Ordering helps readers identify which functions they can call and to find the constructor and fallback definitions easier.

Order of Layout

Layout contract elements in the following order:

  1. Pragma statements
  2. Import statements
  3. Interfaces
  4. Libraries
  5. Contracts

Inside each contract, library or interface, use the following order:

  1. Type declarations
  2. State variables
  3. Events
  4. Functions

Ordering of Functions

Functions should be grouped according to their visibility and ordered:

  1. constructor
  2. receive function (if exists)
  3. fallback function (if exists)
  4. external
  5. public
  6. internal
  7. private

Explicitly label the visibility of functions and state variables. Functions can be specified as being external, public, internal, or private. Please understand the differences between them.

Ordering of Modifiers

The modifier order for a function should be:

  1. Visibility
  2. Mutability
  3. Virtual
  4. Override
  5. Custom modifiers


Documentation & Natspec

Solidity contracts can have a form of comments that provide rich documentation to others reading the code as well as end-users. This is called the Ethereum Natural Language Specification Format, or NatSpec.

This documentation is segmented into developer-focused messages and end-user-facing messages. These messages may be shown to the end-user (the human) at the time that they will interact with the contract (i.e. sign a transaction).

It is recommended that Solidity contracts are fully annotated using NatSpec for all public interfaces (everything in the ABI).



Reading code should be like reading this blog, an article, or a book. It should be well formatted. The Solidity style guide provides guidance for writing Solidity code. The goal for it is not to be the right or only way, but to be consistent.

Consistency can be enforced by using a linter. This not only provides formatting and style guide validation but also includes security validation.

Linters for Solidity

Use a regular linter for your dapp development (e.g. eslint, depending on your language).


Contracts & Data structures

  • Contracts expose behavior and hide data (in abstraction. Not actually hiding data from others. No data on the blockchain is actually hidden, see chapter Contracts). This makes it easy to add new kinds of objects without changing existing behaviors. It also makes it hard to add new behaviors to existing objects.

  • Data structures expose data and have no significant behavior. This makes it easy to add new behaviors to existing data structures but makes it hard to add new data structures to existing functions.

Clear separation of the two is important, due to the contract’s immutability. If you want to add logic or deploy new contracts, you want to be able to utilize the existing data structures. Separating the two can be done by storage contracts. We’ll cover these in more detail in another blog post.


Keep it Simple Stupid ('KISS'). Complexity increases the likelihood of errors and unexpected behavior. Contracts should have one responsibility, and only one reason to change, supporting the Single Responsibility Principle. It's one of the most important concepts in OO design.

Keep your contracts small. Every line of code in a smart contract costs money to execute and storing data is expensive. A best practice is to store a pointer to data in a smart contract, rather than storing the data itself. For example, data can be stored using decentralized data storage providers such as IPFS.

Modularize your contracts to keep them small and utilize existing standards & libraries (see 'Standards & Libraries').

The same applies to functions. Functions should do one thing. They should do it well. They should do it only.

On blockchain everything is public. Private data and variables within a smart contract are not actually private. "Private" means within the execution scope of the contract, but the data is public and can be read by anyone. Keep this in mind when designing your programs.

Only use blockchain and smart contracts for the parts that need to be decentralized.

"The first rule of functions is that they should be small. The second rule of functions is that they should be smaller than that." - Robert C. Martin

Standards & Libraries

An intrinsic feature of smart contracts is its composability. Turning every contract into a component and potential building block that others can leverage. A lot of these building blocks exist already and standards ensure the ease of use and development. The most well-known standard was created for the emerging token ecosystem and the exchanges serving it: the ERC20 standard.

Before you dive in and start writing your own, custom smart contracts, it’s wise to check if any standards or open-source components exist. If such standard contracts exist and are properly tested and audited, they can minimize the risk in your programs.

Error handling

Exceptions are a crucial part of modern software engineering. They require special treatment and can be extremely powerful. Instead of fighting them, we should try to leverage them, as an unhandled exception may cause unexpected behavior.

When an exception is thrown in Solidity code, all state changes are rolled back and further execution is halted. Luckily, exception handling is built into Solidity (since 0.6). This is only available for external calls.

Provide enough context with your exception to determine the source and cause of your error.

Try to write tests that throw these exceptions.

Don’t return null. Don’t pass null to functions.

Guard Checks can help that the behavior of a smart contract and its input parameters are as expected.

Use assert(), require(), revert() properly. assert should be used to test for internal errors, and check invariants. require should be used to test that inputs, contract state variables, or return values from external contracts are valid.

If a transaction doesn’t have enough gas to execute, the out of gas error is not caught.


Test my code

The same rules for clean code, apply to testing. Clean tests are tests that are easy to read. Easy to understand. And easy to maintain. Tests help you keep your code flexible, maintainable, and reusable. They validate the behaviors, reduce the risks of making unintended changes, and save time debugging and writing your code.


  • Fast — Test should fast enough so that you want to run them frequently. It should help speed up your development process, not hinder it
  • Independent — Tests should be independent of each other and able to run in any order
  • Repeatable — Tests should be repeatable in any environment
  • ** Self-validating** — Tests should a clear (boolean) output if it passes.
  • Timely — Tests need to be written in a timely fashion. Just before the production code.


The AAA (Arrange-Act-Assert) pattern is one of the most common standards in software testing. It suggests dividing your tests into the corresponding sections: arrange, act, and assert. Each one of them is responsible for the part in which they are named after.

  • Arrange - The code required to set-up your test
  • Act - Invocation of the method being tested
  • Assert - Check whether the expectations are met

Use only one assert per test.

Separation of tests

  • Unit tests determine whether an individual unit of source-code is fit for use. This is typically a single function, tested with different parameters to ensure it returns the expected values.
  • Integration tests determine if independently developed units of software work correctly when they are connected. For smart contracts, this means interactions between different components of a single contract, or across multiple contracts.

Test coverage

Writing good tests to ensure your app is working as intended is not enough. If you're not measuring anything, it's hard to say exactly how much of your code is being tested. Test coverage (or code coverage) is a useful tool for finding untested parts of a codebase.

There is much debate over how much should be covered. Just your best judgment, but aim for as high as possible. Untested code could do anything and result in unexpected behavior.

Testing Tools

  • Buidler - A task runner for Ethereum smart contract developers that allows you to deploy your contracts, run your tests and debug your code
  • OpenZeppelin Test environment - Test Environment is a smart contract testing library: it provides utilities that help you write tests, but it doesn’t run your tests for you. Works with Mocha
  • Brownie - A Python-based development and testing framework for smart contracts targeting the Ethereum Virtual Machine
  • Truffle - A development environment, testing framework and asset pipeline for blockchains using the Ethereum Virtual Machine (EVM)
  • Ganache - A personal blockchain for Ethereum development you can use to deploy contracts, develop your applications, and run tests
  • Solidity-coverage (formerly Solcover) - Code coverage for Solidity smart-contracts

Security & code analysis

Static code analysis is the analysis of software that is performed without actually executing the program. In most cases the analysis is performed on some version of the source code. Anlaysis is usually performed by an automated tool (e.g. Sonarqube).

Analysis tools for smart contracts help identify smart contract vulnerabilities. These tools run a suite of vulnerability detectors and print out a summary of any issues found.

Analysis Tools

  • Echidna - Ethereum smart contract fuzzer
  • Manticore - Symbolic execution tool
  • Mythril - Security analysis tool for EVM bytecode
  • Oyente - An Analysis Tool for Smart Contracts
  • Slither - Static Analyzer for Solidity

Gas Optimization

Optimizing gas costs can be a worthwhile endeavor, to reduce costs both for your own deployments, as well as for your end-users. Network usage is progressively increasing, with current gas costs at its peak. As the ecosystem continues to grow, so too will the value of gas optimization. We’ll cover these in more detail in another blog post.

Analyzing your code can be done by using eth-gas-reporter. It prints estimates of gas costs per function, or of each deployed contract.

Continuous Integration

While not part of the clean code principles, continuous integration (‘CI’) has become a fundamental practice for any type software development. One of the key benefits of integrating regularly is that you can detect errors quickly and locate them more easily.

It is based on a set of key principles.

  • Keep a single repository where all the code lives and where anyone can obtain the current and previous versions (e.g. Git)
  • Automate the build process so that anyone can use it to build directly from the sources (e.g. Truffle teams, Travis CI)
  • Automate your tests so that anyone can run a full test suite at any time (see chapter Testing)
  • Make sure anyone can view the results and get the latest executable from the build process

"Continuous Integration doesn’t get rid of bugs, but it does make them dramatically easier to find and remove." - Martin Fowler

Ship all the code

Further Reading

Check out my Clean Contracts repo for supporting code example from each chapter.