Hello everyone, I’ve been experimenting with Mojo for a few days and have really enjoyed learning the language so far. While exploring variable declarations, I encountered a behavior that surprised me. According to the documentation: “Variables explicitly declared with var have block-level scope…

As the docs say, the behaviour of the implicit declaration variable is designed to be consistent with Python. In fact, I suspect the only reason this way of declaring variables exists is to maintain consistency with Python. it would make migration easy without introducing correctness issues in code …

Possibly, though it didn’t actually make migration easier for me. The var declaration feels inconsistent both with Python’s scoping rules and with Mojo’s own implicit declaration behavior. Personally, I’d prefer the language to prioritize internal consistency over Python compatibility here. Most Py…

There were some lengthy discussions related to this and there were two fractions in the community: require explicit block scoped var declaration: makes Mojo less complex prohibits accidental introduction of vars (e.g color vs. colour) no accidental var overrides easier to reason about the code/LL…

Mojo will generally keep Python semantics that doesn’t impact performance or safety.

I appreciate you taking the time to share this thoughtful summary — it’s very helpful. I understand that language design is full of trade-offs, and keeping Mojo approachable for Python developers is clearly a high priority. Requiring the var keyword for explicit declaration reduces accidental bugs …

For reference: Discord , Def, fn and var , [Feature] make `var` required for variable declarations in `fn` functions · Issue #5841 · modular/modular · GitHub This was before fn and def were consolidated into def. IMO, having implicit function scoped variable declarations in Mojo still feels like a…

I completely agree — the current mix feels like a legacy compromise rather than a clean design choice. Go handled implicit declarations elegantly with the walrus operator (:=). It clearly signals “this is a new variable” while keeping the same scoping rules as explicit var declarations. No surprise…

Mojo has a walrus operator: modular/mojo/docs/manual/operators.mdx at daa47bb846cc213723a54c51844ea4e923eb5e13 · modular/modular · GitHub But it is limited to expressions, not allowed in statements.

Thanks for the quick reply — I wasn’t aware of Mojo’s walrus operator. while (name := input("Name or ‘quit’: ")) != “quit”: print(“Hello,”, name) This is exactly how the walrus operator is used in Python. My main point is that this creates an implicit declaration of name, which feels inconsisten…

This is a great observation, Kilian, and it hits on one of the most critical design paradigms in Mojo right now. Perhaps a helpful and positive way to frame this behavior is to view the var keyword not just as a syntax requirement, but as an optimization constraint. When we omit var, we are essent…

Block scope vs. function scope of variables

Mojo

christoph_schlumpf (Christoph Schlumpf) June 7, 2026, 6:29am 20

I started this separate forum topic to concentrate on options to require explicit var declarations: Provide an option to require explicit `var` declarations

Topic		Replies	Views
Provide an option to require explicit `var` declarations Mojo 1.0 Beta discussion	24	748	July 31, 2026
Requiring `var` for variable declarations Mojo	4	266	July 28, 2026
Proposal: Make ':=' the standard declaration operator in Mojo (Go-style) Mojo discussion	16	440	June 9, 2026
Why Variable Declaration Matters — And How Mojo Can Become Truly Python-Friendly Mojo discussion	4	278	June 17, 2026
Def, fn and var Mojo discussion	8	611	March 10, 2025

Block scope vs. function scope of variables

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