fix: mo.status.progress_bar correctly handles range step

[z3r0-815]

Fixes #9575

---

mo.status.progress_bar(range(0, 10, 2)) shows impossible progress states. The bug is in how progress_bar calculates the increment per iteration when the range has a non-default step.

Root Cause

progress_bar.__init__ extracts self.step from range objects:

if isinstance(collection, range):
    self.step = cast(range, collection).step

Then __iter__ and __aiter__ use self.step as the increment:

self.progress.update(increment=self.step)

While len(range(0, 10, 2)) correctly computes total = 5, the increment of 2 per iteration over 5 iterations produces current = 10 — double the actual count. The progress bar shows impossible states like 8/5, 10/5.

Fix

• Remove self.step field entirely
• Always increment by 1 per iteration

A progress bar counts loop iterations, not range values. range.step determines what values are yielded, not how many times the loop body executes. Each yield = 1 unit of progress, regardless of the range step. This is consistent with how tqdm behaves — it counts iterations, not step increments.

Tests Added

Test	Input	Items yielded	current after
test_progress_bar_range_with_step	range(0, 10, 2)	[0, 2, 4, 6, 8] (5)	5
test_progress_bar_range_no_step	range(10)	[0..9] (10)	10
test_progress_bar_range_custom_step	range(5, 20, 3)	[5, 8, 11, 14, 17] (5)	5
test_progress_bar_range_negative_step	range(10, 0, -2)	[10, 8, 6, 4, 2] (5)	5

All 17 tests pass (13 existing + 4 new).

Checklist

• Fix is minimal — only the self.step logic removed, increment changed to 1
• All range variants covered: positive step, default step, custom step, negative step
• All existing tests continue to pass
• ruff check and ruff format pass cleanly

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	May 19, 2026 8:50pm

[cubic-dev-ai]

No issues found across 2 files

Architecture diagram

sequenceDiagram
    participant UserCode as User Code
    participant PB as progress_bar()
    participant Progress as _Progress
    participant Output as Output System
    participant Collection as Collection (range/list/etc.)

    Note over UserCode,Collection: Runtime Execution Flow

    UserCode->>PB: Call progress_bar(range(0, 10, 2))
    PB->>PB: Detect collection type (range)
    PB->>PB: Compute total = len(range(0, 10, 2)) → 5
    PB->>PB: Set increment = 1 per iteration
    PB->>Progress: Create ProgressBar instance (total=5)
    Progress-->>UserCode: ProgressBar iterator

    loop For each iteration (5 total)
        UserCode->>Progress: Request next item
        Progress->>Collection: Get next value
        Collection-->>Progress: Yield item (e.g., 0, 2, 4, 6, 8)
        Progress->>Progress: Increment current by 1
        alt Progress enabled
            Progress->>Output: update(current, total)
            Output-->>Progress: Rendered
        end
        Progress-->>UserCode: Yield item
    end

    UserCode->>Progress: Signal end of iteration
    Progress->>Progress: Mark complete (current == total)
    Progress->>Output: Final progress display (5/5)

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[z3r0-815]

I have read the CLA Document and I hereby sign the CLA

[mscolnick]

thank you. we already have a PR up for this #9582