How do pytest fixture dependencies and scopes work, and how do you choose a scope?

Explains pytest fixture dependency resolution, caching, lifetime, and the isolation cost of wider scopes.

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A pytest test requests fixtures by naming them as parameters, and a fixture can request other fixtures the same way. Pytest resolves that dependency graph, creates each fixture once within its scope, and reuses its value for requests inside that scope. I default to function scope because every test gets fresh state. I widen scope only for expensive resources that are safe to share.

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Explains pytest fixture dependency resolution, caching, lifetime, and the isolation cost of wider scopes.

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Explains pytest fixture dependency resolution, caching, lifetime, and the isolation cost of wider scopes.
A pytest test requests fixtures by naming them as parameters, and a fixture can request other fixtures the same way. Pytest resolves that dependency graph, creates each fixture once within its scope, and reuses its value for requests inside that scope. I default to function scope because every test gets fresh state. I widen scope only for expensive resources that are safe to share.
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Scope controls lifetime, not just speed: function, class, module, package, and session scope progressively share more state. A session-scoped database server can be reasonable, while each test still needs its own schema, transaction, or data namespace. Wider mutable fixtures create order dependence unless a narrower fixture resets or partitions their state. I keep dependencies visible instead of hiding important setup behind broad autouse fixtures.

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Making every expensive resource function scoped
Making mutable test data session scoped
Using autouse fixtures that hide important state

How do pytest fixture dependencies and scopes work, and how do you choose a scope?

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