Resolve import loops

[EarlMilktea]

Context (if applicable):

This PR fixes another import loop causing problems in #280 .

Description of the change:

• Resolve import loops caused by prety_print.py
• Enforce top-level imports by ruff
  • Some exceptions exist: see pyproject.toml .

After merging this PR, for all the typed files, import will become completely loop-free.

[codecov]

Codecov Report

Attention: Patch coverage is 96.51163% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.82%. Comparing base (1dc02d7) to head (28ad8cf).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
graphix/command.py	96.92%	2 Missing ⚠️
graphix/pattern.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## master #301 +/- ##
==========================================
- Coverage 79.83% 79.82% -0.01% 
==========================================
Files 36 36 Lines 5429 5428 -1 ==========================================
- Hits 4334 4333 -1 
Misses 1095 1095 

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[thierry-martinez]

I would be very disappointed if pattern_to_str could not remain in pretty_print.py (and, to a lesser extent, the same applies to command_to_str):

• We should reduce the size of the pattern.py module.

• I would like to clearly separate data structure definitions (patterns, commands) from algorithmic operations (pretty-printing, optimization, simulation, visualization), with each operation having its own dedicated module.

[EarlMilktea]

@thierry-martinez

I really understand your opinion but still import loop is what we should not have.
It is ridiculous that I see esoteric errors each time I change the order of import .

clearly separate data structure definitions from algorithmic operations
pattern_to_str remain in pretty_print.py

This could be possible if Pattern is downsized. I'm happy with suspending this PR until ready.

command_to_str

This is 100 % impossible since it immediately results in import loop.

Let's find our point of compromise.

[thierry-martinez]

I agree with you that we should consider import loops to be bugs, and we want to resolve them so that modules can be imported in any order without causing random failures.

I propose in #304 a lighter fix to resolve the import loop between pretty_print and command. In this PR, you also move the statement from graphix.sim.density_matrix import DensityMatrix to the top level of random_objects.py. That seems correct, and I can’t explain why I felt the need to move it elsewhere in commit a9493b7, since it did not appear to be necessary, so we can keep your fix for this in the PR.

[EarlMilktea]

@thierry-martinez

Checked #304 .

While it would be sufficient for now, I still strongly believe that, at least in the future, we need to consider loop-free (DAG) import structure and actually this is possible by transforming xxx_to_str to methods.
I admit your design (separate data structure definitions from algorithmic operations) is very common and idiomatic in some cases (for example as Rust does), but specifically for Python, class-based encapsulation (implement class functionalities locally inside the class) is also equally idiomatic and this time clearly superior to yours as it forever eliminates subtle import-order errors.

I hope we someday reach agreement.

[thierry-martinez]

Python doesn't forbid cycles in the dependency graph, only import-time execution cycles are problematic. Moreover, the key issue isn’t whether something is implemented inside a class or a function: object-oriented design can be helpful in many cases.

The deeper problem lies in having a large, central module like pattern, which is inherently core to graphix. This makes it tempting to place a lot of functionality there. But that’s a common pitfall when aiming for a strict DAG design: it tends to create “god modules” that accumulate too much responsibility. Instead, I believe we should aim for better separation of concerns between distinct functionalities.

Conversely, naive object-oriented design can also lead to fragmentation: spreading the implementation of a single functionality across multiple classes or modules. Grouping related algorithmic parts can actually improve cohesion and clarity. For example, having a dedicated pretty_print module that sets the conventions for rendering both patterns and commands helps ensure consistency and improves maintainability. In that sense, keeping the formatting logic centralized is a benefit, not a flaw.

[EarlMilktea]

This could be possible if Pattern is downsized. I'm happy with suspending this PR until ready.

As I said I'm ready for delaying this PR so that Pattern does not becomes too "divine" now.
What I want to confirm here is, we should someday ensure loop-free import structures statically by PLC0415 , so that this kind of random failures never occur in the future.

[thierry-martinez]

The rule PLC0415 could be desirable (and we can easily comply with it; see #307), but it does not require a DAG structure. As I said, only import-time execution cycles are problematic, and I think that requiring a DAG structure will reduce the code’s modularity and maintainability by forcing things to be grouped or separated for purely technical reasons, regardless of the logic of the code.

[EarlMilktea]

will reduce the code’s modularity and maintainability by forcing things to be grouped or separated for purely technical reasons, regardless of the logic of the code

I think, it is still preferable to random breakdowns.

[thierry-martinez]

We agree that import-time execution cycles should be considered as bugs, and we want to resolve them so that modules can be imported in any order without causing random failures. PR #304 resolves a bug preventing graphix.pretty_print from being imported first. PR #307 makes the whole code base satisfy PLC0415 by moving all imports to the top of modules. Do you approve these PR?