What is the difference between a shallow and deep copy in Python?

Compares copying strategies through nested state, ownership, performance, and domain identity rather than presenting deep copy as universally safer.

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TL;DR

Assignment creates another reference; it does not copy the object. A shallow copy creates a new outer container but keeps references to the same nested objects. A deep copy recursively copies reachable contents, using memoization to preserve repeated references and handle many cycles. If I shallow-copy a list of mutable dictionaries, replacing an outer list item is independent, but changing a shared dictionary is visible through both lists.

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Understanding that assignment does not copy, shallow copies share nested objects, and deep copies carry semantic and operational costs.

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Core answer
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Overview
Copy behavior matters most when a structure contains nested mutable values or identities that should not be duplicated.
Assignment creates another reference; it does not copy the object. A shallow copy creates a new outer container but keeps references to the same nested objects. A deep copy recursively copies reachable contents, using memoization to preserve repeated references and handle many cycles. If I shallow-copy a list of mutable dictionaries, replacing an outer list item is independent, but changing a shared dictionary is visible through both lists.
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Stronger answer
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Shallow copy is appropriate when I need a separate container while nested values are immutable or intentionally shared. Deep copy can be useful for a genuinely independent object graph, but it may be expensive and may duplicate objects whose identity matters. Types can customize copy behavior, and resources such as file handles, locks, database sessions, and network clients are not meaningful candidates for generic recursive duplication.

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Details

Assuming assignment copies an object.
Expecting a shallow copy to isolate nested mutable values.
Using deep copy on sessions, locks, or external resources.
Ignoring identifiers and shared domain entities in a copied graph.

What is the difference between a shallow and deep copy in Python?

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