fix(client): give specific error for blank/whitespace pathspec components

[dheerenmohta]

Problem

When a pathspec contains a whitespace-only component (e.g. "Flow/ /Step"), the existing code produces a confusing error:

• If the whitespace-only component causes a wrong count → the error says "Expects Run(...)" — no mention of the blank segment
• If there are the right number of components → no error at all (whitespace passes str.split("/") silently)

In both cases the user has no clear signal about what is wrong.

Fix

Add an explicit check before the component-count validation that:

1. Finds all blank or whitespace-only component positions
2. Raises MetaflowInvalidPathspec with the offending position(s) called out

blank = [i for i, c in enumerate(ids) if not c.strip()]
if blank:
    raise MetaflowInvalidPathspec(
        "Pathspec '%s' has an empty or blank component at position(s) %s. ..."
        % (pathspec, ...)
    )

No behaviour change for valid pathspecs.

Fixes #948 (partial — error message quality)

[greptile-apps]

Greptile Summary

This PR adds an explicit check for blank or whitespace-only components in a pathspec string before the existing length validation, raising MetaflowInvalidPathspec with the affected position(s) clearly called out. Valid pathspecs are unaffected.

• Inserts a list comprehension to collect 0-indexed positions of blank components from ids and raises a descriptive error immediately, so users get actionable feedback instead of a confusing "Expects Run(...)" message or silent acceptance.
• Covers both empty ("Flow//Step") and whitespace-only ("Flow/ /Step") components, as well as leading/trailing slashes and wholly-blank pathspecs.

Confidence Score: 4/5

Safe to merge; the change is a small guard that fires only when a pathspec contains a blank component, leaving all valid pathspecs completely unchanged.

The fix is logically correct and well-placed, with one minor UX note: positions in the error message are 0-indexed, which may be slightly confusing for users who think of path components as starting at 1.

No files require special attention beyond the single changed guard in metaflow/client/core.py.

Important Files Changed

Filename	Overview
metaflow/client/core.py	Adds a blank/whitespace component check before the length validation in MetaflowObject.init, raising MetaflowInvalidPathspec with the offending 0-indexed position(s). Logic is correct; one minor UX note on 0-indexed position reporting.

_{Reviews (1): Last reviewed commit: "fix(client): improve error message for b..." | Re-trigger Greptile}

[greptile-apps]

The positions are reported as 0-based indices, but end-users mentally count path components from 1 (e.g., "Flow/ /Step" — component 2 is blank, not component 1). Using 1-based positions in the error message would be more intuitive here.

Suggested change

	raise MetaflowInvalidPathspec(
	"Pathspec '%s' has an empty or blank component at position(s) %s. "
	"Each '/' must separate non-empty path parts."
	% (pathspec, ", ".join(str(p) for p in blank))
	)
	raise MetaflowInvalidPathspec(
	"Pathspec '%s' has an empty or blank component at position(s) %s. "
	"Each '/' must separate non-empty path parts."
	% (pathspec, ", ".join(str(p + 1) for p in blank))
	)

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@dddc0d5). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
metaflow/client/core.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3261   +/-   ##
=========================================
  Coverage          ?   29.08%           
=========================================
  Files             ?      381           
  Lines             ?    52516           
  Branches          ?     9267           
=========================================
  Hits              ?    15275           
  Misses            ?    36204           
  Partials          ?     1037           

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