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Summary

Changes Python session state preservation to be the default behavior in mpremote. Previously, mpremote performed automatic soft-reset between command sessions, clearing variables and imports. Users had to explicitly use the resume command to preserve state.

This change makes the "resume" behavior the default and removes the resume command entirely. Users wanting the old auto soft-reset behavior can now explicitly add soft-reset to their command sequences.

Example behavior change:

# Before (auto soft-reset default):
mpremote exec "a = 1" eval "a"  # Would fail with NameError
mpremote resume exec "a = 1" eval "a"  # Would work

# After (resume default):
mpremote exec "a = 1" eval "a"  # Now works by default
mpremote soft-reset exec "a = 1" eval "a"  # Explicit soft-reset for old behavior

Testing

  • Updated existing test_resume.sh test to reflect new behavior
  • All pre-commit hooks pass (ruff, codespell, codeformat.py)
  • Tested that explicit soft-reset commands replicate the old default behavior

Trade-offs and Alternatives

Benefits:

  • More intuitive behavior - variables persist as users expect
  • Reduces need for explicit resume commands in workflows
  • Simplifies automation scripts that work with persistent state

Breaking change considerations:

  • Existing scripts expecting auto soft-reset will see different behavior
  • Some users may have automation that depends on clean state per command
  • Migration path: add explicit soft-reset where clean state is required

Implementation details:

  • Changed State.__init__() to set _auto_soft_reset = False by default
  • Removed do_resume() function and command registration
  • Removed auto-reset on disconnect to maintain consistency
  • Explicit soft-reset command continues to work as before

This provides the same functionality as before but with inverted defaults, making the more commonly desired behavior (state persistence) the default while still allowing users to get clean state when needed.

@andrewleech andrewleech marked this pull request as draft June 13, 2025 09:22
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Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32

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codecov bot commented Jun 13, 2025

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (2ce63b1) to head (901cf0d).
Report is 13 commits behind head on master.

Additional details and impacted files
@@           Coverage Diff           @@
##           master   #17485   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files         169      169           
  Lines       21941    21946    +5     
=======================================
+ Hits        21621    21626    +5     
  Misses        320      320           

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@andrewleech
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andrewleech commented Jun 13, 2025

This is prompted by discussions started in #17322 (comment)

For reference this was built from just three prompts in a new Claude code session:

╭──────────────────────────────────────────────╮
│ > for this activity I'm focusing on the mpremote     │
│   package in tools. can you review the command line  │
│   arg "resume" and compare/contrast it to "soft      │
│   -reset", is it just the opposite?                  │
╰──────────────────────────────────────────────╯
● Based on my analysis of the mpremote code, here's the               20:09:29 [475/525]
  comparison between resume and soft-reset:

  resume vs soft-reset - Not Exact Opposites

  resume command (commands.py:578-581):
  - Sets state._auto_soft_reset = False
  - Sets state.was_resumed = True
  - Prevents automatic soft reset when entering raw REPL                                  - Preserves the Python interpreter state from previous
  session

  soft-reset command (commands.py:583-585):
  - Calls state.ensure_raw_repl(soft_reset=True)
  - Explicitly performs a soft reset of the MicroPython
  interpreter
  - Resets all variables and imported modules
  - Calls state.did_action() to mark that an action was
  performed

  Key Differences

  1. Purpose:
    - resume prevents automatic resets to continue where
  you left off
    - soft-reset explicitly resets the interpreter state
  2. State preservation:
    - resume preserves variables and imports from
  previous commands
    - soft-reset clears all variables and reloads the
  interpreter
  3. Default behavior:
    - By default, mpremote performs auto soft-reset
  between sessions
    - resume overrides this default to maintain
  continuity
    - soft-reset explicitly triggers what normally
  happens automatically

  The test file demonstrates this: after exec "a =
  'resume'", the resume eval "a" command can access the
  variable a, while normally a new session would trigger
  a NameError because the variable would be cleared by
  the automatic soft reset.

  So while they are related to reset behavior, resume is
  more about preventing resets rather than being the
  opposite of soft-reset. It's more accurate to say
  resume disables automatic soft-reset rather than being
  its inverse.

I started with that above to encourage Claude to review the code around the area of interest, I then followed up with a more specific request:

╭───────────────────────────────────────────────────╮
│ > I would like to make the "resume" behaviour the default  │
│   so that users get this automatically. this would         │
│   involve removing the resume command altogether. if       │
│   users then wanted to get the soft-reset on connect       │
│   behaviour back (as is the default currently) would       │
│   explicitly adding soft-reset to their mpremote commands  │
│   line be essentially the same as the current default      │
│   behaviour?                                               │
╰───────────────────────────────────────────────────╯

Which returned:

● Based on my analysis, yes, explicitly adding soft-reset
   to the command line would essentially replicate the
  current default behavior. Here's why:

  Current Behavior Analysis

  Current Default (auto soft-reset = True):
  1. State.__init__() sets _auto_soft_reset = True
  (main.py:501)
  2. Commands calling ensure_raw_repl() trigger auto
  soft-reset on first connection
  3. After first soft-reset, _auto_soft_reset becomes
  False (main.py:521)
  4. On disconnect, _auto_soft_reset resets to True
  (commands.py:184)

  Resume Command Current Behavior:
  - Sets _auto_soft_reset = False immediately
  (commands.py:579)
  - Sets was_resumed = True for reconnect persistence
  (commands.py:580)

  Proposed Change Impact

  If you make "resume" the default behavior:

  1. Change: Set _auto_soft_reset = False in
  State.__init__()
  2. Remove: The resume command entirely
  3. User override: Users add explicit soft-reset to get
  current default

  Example usage patterns:

  # Current default behavior (with explicit soft-reset):
  mpremote soft-reset exec "a = 1" eval "a"  # a = 1

  # New default behavior (resume-like):
  mpremote exec "a = 1" eval "a"             # a = 1

  Key Considerations

  ✅ Would work because:
  - Explicit soft-reset calls
  ensure_raw_repl(soft_reset=True) regardless of
  _auto_soft_reset value
  - Commands that need raw REPL would preserve state
  between invocations
  - Reconnection logic already handles was_resumed state
  properly

  ⚠️  Backwards compatibility concerns:
  - Existing scripts expecting auto soft-reset would see
  different behavior
  - Users might get confused variables persisting between
   sessions
  - Some automation scripts might depend on clean state
  per command

  📝 Migration path:
  - Users wanting old behavior: add soft-reset at start
  of command sequences
  - Scripts could be updated incrementally
  - Clear documentation on the behavior change

  The explicit soft-reset command would indeed replicate
  the current default behavior because it bypasses the
  _auto_soft_reset flag and forces a reset regardless of
  the state.

So I asked:

╭───────────────────────────────────────────╮   
│ > checkout master, then checkout a new branch     │
│   for this change, then make the change in code,  │   
│   commit the change, push to GitHub, raise a PR   │   
│   with that information/summary just prepared     │
╰───────────────────────────────────────────╯


and that was enough to get the PR up here.

Of note; it gets commits and pr template basically right thanks to my current:
CLAUDE.md

TODO test on hardware to ensure it behaves as expected, after which I'll mark it as ready for review

Changes Python session state preservation to be the default behavior.
Previously, mpremote performed auto soft-reset between command
sessions, clearing variables and imports. Users had to explicitly
use 'resume' to preserve state.

This change:
- Sets _auto_soft_reset = False by default in State.__init__()
- Removes the 'resume' command entirely
- Updates tests to reflect new default behavior

Users wanting the old auto soft-reset behavior can now explicitly
add 'soft-reset' to their command sequences.

Breaking change: Existing scripts expecting auto soft-reset will see
different behavior. Migration: add explicit 'soft-reset' commands
where clean state is required.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>
@andrewleech andrewleech force-pushed the mpremote-resume-by-default branch from 5608292 to 901cf0d Compare June 13, 2025 09:55
@Gadgetoid
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Gadgetoid commented Jun 13, 2025

Co-Authored-By: Claude <noreply@anthropic.com>

Is this a meaningful thing to add? A tool is not an author and a "noreply" email isn't much use for posterity.

Edit: To engage with the change itself - if you wanted a repl wouldn't you just use a repl? I would argue that state persisting between individual mpremote commands would be the more surprising outcome, since mpremote is - to my understanding - intended to get one thing done and then clean up after itself. mpremote repl notwithstanding.

@dpgeorge dpgeorge added the tools Relates to tools/ directory in source, or other tooling label Jun 16, 2025
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I'm in favour of this change. I think it makes mpremote more consistent, in the following cases:

  1. mpremote repl does not do a soft reset.
  2. mpremote run a.py run b.py does not do a soft reset for the second run.

Also, in the cases where the connection is lost on soft reset (eg with a dynamic USBDevice, or WebREPL) it makes sense for a soft reset to be an explicit command, because you usually don't want it.

@Josverl
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Josverl commented Jun 16, 2025

I use mpremote a lot through subprocess run in mpflash, micropython-magic, end other tools.
I would say that over 90% of the time I have needed to inserting resume in each command line in order work around the current behavior. ( I know I should not wrap mpremote ... ) Once built that is simple for a tool to keep doing, but I see users running into issues as well.

I see resetting as a side effect that should only happen if requested.
but as muscle memory sometimes is hard to retrain, perhaps there should be a way to keep the old behavior ?
perhaps though an environment variable MPREMOTE_AUTO_RESET=1 ?

@andrewleech
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Is this a meaningful thing to add? A tool is not an author and a "noreply" email isn't much use for posterity.

I'm very open to discussions about this, at the moment I'm just trying to be very up front about my use of the tool.

I'd kinda argue the tool was the primary author and I'm just the reviewer. Sure I provided the description of the change I wanted, but I never wrote a single line of code here.

Regardless, the coauthor tag is recognised by GitHub such that it shows prominently that Claude was used which I think it's a good thing to be open about; though it's true I'm still the person responsible for submitting the change request.

@andrewleech
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if you wanted a repl wouldn't you just use a repl? I would argue that state persisting between individual mpremote commands would be the more surprising outcome, since mpremote is - to my understanding - intended to get one thing done and then clean up after itself. mpremote repl notwithstanding.

99% of my usage of mpremote is to attach to a device already running code, with aiorepl running in the codebase. It's very rare I want it to reboot the device on me, unless I actually send the reboot or bootloader command explicitly.

I'm generally using micropython to build products, so have applications running from main pretty much all the time outside of small research / feature development spikes or separate micropython pr development efforts.

So your suggestion of it being surprising that a device would keep running between mpremote calls is curious to me; I'd be interested to hear more about your workflow that lead to this expectation? I don't want to make mpremote more confusing for people!

@Gadgetoid
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So your suggestion of it being surprising that a device would keep running between mpremote calls is curious to me

I think we come at this from wildly different angles and I spend 99% of my time deep in the weeds writing C modules, and most of my higher level development time ardently dogfooding Thonny.

My uses of mpremote start and end with mpremote mip where I'll always want a clean slate afterwards, and potentially other one off prods 'n' pokes. In this case I have to defer to your expertise!

it shows prominently that Claude was used

Feels like a delicate balance between an advertisement for Claude and an admission that it's useful to know what it's touched just in case it proves to be so radioactive that it has to be chopped out of the codebase 😆 It's also a tacit approval of AI that could open the floodgates to much less thoughtful use of it than yours (uh that ship might have sailed without my noticing, to be fair). Suffice to say my takes on AI contributions to code are not optimistic. (understatement of the year, since I don't believe even thoughtful use is productive beyond the most crude and short term measures)

Fundamentally Claude can't really "own" a commit in any meaningful sense. AIUI It doesn't have institutional knowledge beyond that enshrined in the code itself, nor can it consistently or reliably hold a viewpoint or belief and advocate for a particular decision. Even if I fed it your/its original rational/analysis/prompt. Thus it functionally cannot be an author or a meaningful, discrete part of the fleshy machine that drives forward a FOSS project.

That is to say- If I were to question Claude about this change and posit an alternative, I suspect it would immediately capitulate to whatever I thought was best 😆 If I were to question you... well, you have successfully given rationale, and argued your corner.

Anyway, TLDR: there are probably better places for this discussion, or perhaps conversations I have already missed. As you were!

@andrewleech
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Suffice to say my takes on AI contributions to code are not optimistic.

I was the same only a few months ago, my belief was they were mildly helpful in a very narrow set of use cases. Since I got access to Claude Code my entire professional way of working has been turned upside down, this tool is redefining my job role and more than doubling my productivity and quality of work.

Thus it functionally cannot be an author or a meaningful, discrete part of the fleshy machine that drives forward a FOSS project.

This is a useful angle to think of certainly. At work the crucial point of view is that regardless of how instrumental AI might be in producing a piece of work I, and the company, have to take ownership it from a responsibility point of view and have to stand by it being as good (if not better) then work I would have done the manual way.

Anyway, TLDR: there are probably better places for this discussion, or perhaps conversations I have already missed. As you were!

While yes it's off topic to the code of the PR I did prompt this discussion in my first comment... though maybe there should be a formal discussion started elsewhere to discuss some policies around appropriate declarations of AI use that's informative without being just advertisement.

@andrewleech
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mpremote run a.py run b.py does not do a soft reset for the second run.

I was getting unsure whether a reset before run would be more intuitive still, but this case is interesting.

connection is lost on soft reset (eg with a dynamic USBDevice, or WebREPL)

Avoiding the implicit reset for the cases where connections are broken is the one I keep coming back to as a stronger argument for this change; it would be nice to change soft reset enough to never break connections but these ones in particular would be really hard to achieve.

@dpgeorge
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dpgeorge commented Jul 1, 2025

Another argument for making resume the default: currently if you do:

$ mpremote exec "x=1" exec "print(x)"

then that works as expected, it sets and prints x.

But if you run the exec's separately:

$ mpremote exec "x=1"
$ mpremote exec "print(x)"

then it doesn't work because the second time it does a soft reset and loses all variables.

If we make resume the default, then both the above examples work the same.

I think mpremote run is the only command where you might expect it to do a soft reset first. So maybe we can have two run commands, one which does soft reset first, and one which does not?

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