---
title: ParallelizationTrait
framework: testing
role: symbol
role_heading: Structure
path: testing/parallelizationtrait
---

# ParallelizationTrait

A type that defines whether the testing library runs this test serially or in parallel.

## Declaration

```swift
struct ParallelizationTrait
```

## Overview

Overview When you add this trait to a parameterized test function, that test runs its cases serially instead of in parallel. This trait has no effect when you apply it to a non-parameterized test function. When you add this trait to a test suite, that suite runs its contained test functions (including their cases, when parameterized) and sub-suites serially instead of in parallel. If the sub-suites have children, they also run serially. This trait does not affect the execution of a test relative to its peers or to unrelated tests. This trait has no effect if you disable test parallelization globally (for example, by passing --no-parallel to the swift test command.) To add this trait to a test, use serialized.

## Relationships

### Conforms To

- [Copyable](swift/copyable.md)
- [CustomStringConvertible](swift/customstringconvertible.md)
- [Escapable](swift/escapable.md)
- [Sendable](swift/sendable.md)
- [SendableMetatype](swift/sendablemetatype.md)
- [SuiteTrait](testing/suitetrait.md)
- [TestScoping](testing/testscoping.md)
- [TestTrait](testing/testtrait.md)
- [Trait](testing/trait.md)

## See Also

### Supporting types

- [Bug](testing/bug.md)
- [Comment](testing/comment.md)
- [ConditionTrait](testing/conditiontrait.md)
- [IssueHandlingTrait](testing/issuehandlingtrait.md)
- [Tag](testing/tag.md)
- [Tag.List](testing/tag/list.md)
- [TimeLimitTrait](testing/timelimittrait.md)