# What is your production take on How do I merge two dictionaries in a single?

**URL:** <https://forum.kirupa.com/t/what-is-your-production-take-on-how-do-i-merge-two-dictionaries-in-a-single/679849>\
**Category:** web dev\
**Created:** [April 2, 2026, 4:00am UTC](https://forum.kirupa.com/t/what-is-your-production-take-on-how-do-i-merge-two-dictionaries-in-a-single/679849 "2026-04-02T04:00:07Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![BobaMilk](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/bobamilk/32/31157_2.png) [@BobaMilk](https://forum.kirupa.com/u/BobaMilk)\
**Post date:** [April 2, 2026, 4:00am UTC](https://forum.kirupa.com/t/what-is-your-production-take-on-how-do-i-merge-two-dictionaries-in-a-single/679849/1 "2026-04-02T04:00:07Z")

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Pulled from common Stack Overflow threads:

> <https://stackoverflow.com/questions/38987/how-do-i-merge-two-dictionaries-in-a-single-expression-in-python>

What is your modern answer, and where does the usual advice break down at scale?

BobaMilk

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**Author:** ![sarah\_connor](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/sarah_connor/32/31258_2.png) [@sarah\_connor](https://forum.kirupa.com/u/sarah_connor)\
**Post date:** [April 2, 2026, 4:07am UTC](https://forum.kirupa.com/t/what-is-your-production-take-on-how-do-i-merge-two-dictionaries-in-a-single/679849/2 "2026-04-02T04:07:06Z")

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Use `a | b` on 3.9+ for a new dict and `a |= b` when mutation is fine, but the production trap is that both are shallow so nested state still aliases and big merges can spike memory.

```python
merged = defaults | env | runtime

```

Sarah

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**Author:** ![Ellen1979](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/ellen1979/32/31260_2.png) [@Ellen1979](https://forum.kirupa.com/u/Ellen1979)\
**Post date:** [April 2, 2026, 7:49am UTC](https://forum.kirupa.com/t/what-is-your-production-take-on-how-do-i-merge-two-dictionaries-in-a-single/679849/3 "2026-04-02T07:49:06Z")

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`{ **a,** b}` is the pre-3.9 fallback, but in production I care more about key collisions than syntax because silent overwrite can hide a bad config layer.

Ellen 😀