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Testing smart contracts

Introduction​

Because Tezos blockchain smart contracts are immutable after deployment, you must rigorously test to ensure functionality, prevent errors, and avoid potential financial losses. Importantly, contract testing doesn't require any tokens or a wallet account to execute.

High-level languages come with tools to help write tests, and some testing tools can be used independently of the language used to write the smart contract. For example, SmartPy includes syntax dedicated to testing.

Structure of a test scenario

A test scenario usually consists of the following steps:

  1. Decide the smart contract's initial storage and balance
  2. Valid calls to entrypoints, with different parameters and context information such as:
    • the address of the caller
    • the amount of tez sent
    • the timestamp of the block (value of now during the call)
    • the level of the block
  3. Verify the contract's storage or balance changed the way you expected.
  4. Try some invalid calls that should fail, and they are expected to fail.
  5. Verification of the error caused by these invalid calls, making sure the error messages are the ones you thought would come up.

When executed, the test scenario is successful if all verifications are correct, and all invalid calls fail with the expected errors.

More advanced scenarios may involve a local sandbox deployment and calls to multiple contracts to test interactions.

Programming languages for testing​

The test scenarios are usually written using a full classical programming language, such as JavaScript or Python, with a library that gives you access to special features to:

  • Deploy contracts
  • Make calls to entrypoints
  • Manipulate all the types/values supported by Tezos
  • Generate testing accounts, to simulate calls from multiple accounts
  • Perform cryptographic computations similar to the ones available in the contract

Rules for testing​

Testing a contract thoroughly is not easy and requires experience. Here are some tips to follow when getting started:

  • Write tests without looking at the implementation of the contract to avoid copying mistakes.
  • If possible, have another developer write the test to avoid testing semantic errors incorrectly.
  • Make sure to cover every possible execution path, whether it's valid or invalid.
  • Create many small tests, each checking something very specific, rather than a long test that tries to do many things at once.
  • Test around the limits For example, if a value should be always above 10, include a call with the value 10 that should fail and a call with the value 11 that should succeed.
  • Test extremes

For more information about avoiding flaws in contracts, see Avoiding flaws on opentezos.com.

Implementation details​

Upon test successful, you can deploy your smart contract!