Added JSON serialisation for Lambda functions by bennthomsen · Pull Request #1695 · QCoDeS/broadbean

bennthomsen · 2026-02-02T13:03:22Z

Pull Request Summary

Title: Added JSON serialization for Lambda functions

Description

This PR adds support for serializing and deserializing BluePrint objects that contain arbitrary functions (arb_func), including lambda functions and named functions, to/from JSON format.

Changes

Files Modified:

src/broadbean/blueprint.py (+131 lines) - Core serialization/deserialization logic
tests/test_arb_func_serialization.py (+263 lines) - New comprehensive test suite
tests/test_blueprint.py (+1 line) - Updated existing test

Key Features

Lambda Function Serialization: Lambda functions used with PulseAtoms.arb_func can now be serialized to JSON by storing their source code, extracted using inspect.getsource() with regex parsing.
Named Function Serialization: Regular named functions are serialized with both their name and full source code, allowing reconstruction even in different Python sessions.
Graceful Fallbacks: When functions cannot be reconstructed (e.g., missing dependencies or invalid source), the system falls back to a zero function with appropriate warnings.
Sample Rate Persistence: The SR (sample rate) property is now included in the blueprint description and restored during deserialization.
Backward Compatibility: Legacy JSON formats without the new func_type field are still supported.

Usage Example

import broadbean as bb
import numpy as np

# Create a blueprint with a lambda function using arb_func
bp = bb.BluePrint()

# Add a segment using arb_func with a lambda function
my_lambda = lambda t, **kwargs: np.sin(2 * np.pi * kwargs.get('freq', 1) * t)
bp.insertSegment(
    0, 
    bb.PulseAtoms.arb_func, 
    args=(my_lambda, {'freq': 10}),  # (function, kwargs_dict)
    name='sine_wave',
    dur=1e-6
)
bp.setSR(1e9)

# Serialize to JSON file
bp.write_to_json('my_blueprint.json')

# Later, reconstruct the blueprint from JSON
bp_restored = bb.BluePrint.init_from_json('my_blueprint.json')

# The lambda function is reconstructed and works as expected
print(bp_restored.description)

Example with a named function:

import broadbean as bb
import numpy as np

# Define a custom waveform function
def gaussian_pulse(t, **kwargs):
    amplitude = kwargs.get('amplitude', 1.0)
    sigma = kwargs.get('sigma', 1e-7)
    center = kwargs.get('center', 5e-7)
    return amplitude * np.exp(-((t - center) ** 2) / (2 * sigma ** 2))

# Create blueprint
bp = bb.BluePrint()
bp.insertSegment(
    0,
    bb.PulseAtoms.arb_func,
    args=(gaussian_pulse, {'amplitude': 0.5, 'sigma': 2e-7, 'center': 5e-7}),
    name='gaussian',
    dur=1e-6
)
bp.setSR(1e9)

# Save and restore
bp.write_to_json('gaussian_blueprint.json')
bp_restored = bb.BluePrint.init_from_json('gaussian_blueprint.json')

Technical Details

Serialization (description property):

Detects arb_func segments and extracts function information
For lambdas: extracts source code using regex to isolate the lambda expression
For named functions: stores both function name and complete source code
Stores associated kwargs dictionary

Deserialization (blueprint_from_description):

Reconstructs lambda functions using eval()
Reconstructs named functions by executing source code in isolated namespace
Falls back to global namespace lookup or zero function if reconstruction fails

Testing

Added comprehensive test suite (test_arb_func_serialization.py) covering:

Lambda function serialization round-trip
Named function serialization round-trip
Complex expressions with numpy operations
Kwargs preservation
JSON file write/read operations
Error handling and fallbacks
Integration with existing blueprint functionality

Breaking Changes

None - existing functionality and JSON format remains compatible.

for more information, see https://pre-commit.ci

…oadbean into JSONserialisation

codecov · 2026-02-03T13:33:36Z

Codecov Report

❌ Patch coverage is 42.42424% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.16%. Comparing base (ee22118) to head (93d64c2).

Files with missing lines	Patch %	Lines
src/broadbean/blueprint.py	42.42%	38 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1695      +/-   ##
==========================================
- Coverage   72.12%   71.16%   -0.97%     
==========================================
  Files           9        9              
  Lines        1539     1602      +63     
==========================================
+ Hits         1110     1140      +30     
- Misses        429      462      +33

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astafan8 · 2026-02-03T13:33:10Z

docs/examples/Pulse_Building_Tutorial.ipynb

why are there two duplicated(?) plots instead of one in this example? or is it github mis-rendering?

astafan8 · 2026-02-03T13:35:40Z

src/broadbean/blueprint.py

+        # for ii, name in enumerate(namelist):
+        #     if isinstance(funlist[ii], str):
+        #         namelist[ii] = funlist[ii]
+        #     elif name == "":
+        #         namelist[ii] = funlist[ii].__name__
        for ii, name in enumerate(namelist):
            if isinstance(funlist[ii], str):
-                namelist[ii] = funlist[ii]
+                if name == "":
+                    namelist[ii] = funlist[ii]
            elif name == "":
                namelist[ii] = funlist[ii].__name__


also, what happens now in the else of if isinstance(funlist[ii], str): if name == "":? I am a bit worried about the test coverage of this portion of the code being too low to ensure this change is not breaking anything - what do you think?

why keeping the commented out code part?

astafan8 · 2026-02-03T13:36:20Z

src/broadbean/blueprint.py


-        self._namelist = namelist
        namelist = self._make_names_unique(namelist)
+        self._namelist = namelist


this looks like a good catch! thank you!

astafan8 · 2026-02-03T13:38:23Z

src/broadbean/blueprint.py

+                                func_source = "lambda t, **kwargs: 0"
+                        except (OSError, TypeError):
+                            # Fallback: create a generic lambda string
+                            func_source = "lambda t, **kwargs: 0"  # Default fallback


shouldn't it rather be None, like in case the source code of named function is not possible to retireve? This would make the behavior consistent while having a default fallback can become confusing

astafan8 · 2026-02-03T13:39:37Z

src/broadbean/blueprint.py

+                            if func_name in local_ns:
+                                func_obj = local_ns[func_name]
+                        except Exception as e:
+                            print(


next to printing please use logging.warning so that this information can also show as a proper warning, and be included in logs (if those are used)

astafan8 · 2026-02-03T13:43:07Z

src/broadbean/blueprint.py

+                    # Regular function - store name and try to get source
+                    func_name = func_obj.__name__
+                    try:
+                        func_source = inspect.getsource(func_obj)


isn't it extremely fragile? What is a function is using functions from math module or numpy, would those imports be also part of the source?

also, i can't imagine that noone tried to solve this problem before - can't more functions/methods from the python standard library or perhaps even an external package be used to support this rather than implementing a custom function code serialization here?

astafan8 · 2026-02-03T13:43:51Z

src/broadbean/blueprint.py

+                func_obj, kwargs_dict = self._argslist[sn]
+
+                # Serialize the function
+                if hasattr(func_obj, "__name__") and func_obj.__name__ != "<lambda>":


could you move the logic of extracting code a function into a separate function in order not to mix it with the logic of creating blueprint description?

astafan8 · 2026-02-03T13:45:14Z

src/broadbean/blueprint.py

+                        try:
+                            # Execute the function source in a local namespace
+                            local_ns = {}
+                            exec(func_source, globals(), local_ns)


this is dangerous to make globals available, both from security perspective but also from the perspective of writing code that would prepare some globals to exist before deserializing certain blueprints and their arbitrary functions. Would it be possible to limit support to self-sufficient functions? at least to start with

benn-thomsen and others added 6 commits February 2, 2026 12:23

Added JSON serialisation for Lambda functions

2e7bd34

[pre-commit.ci] auto fixes from pre-commit.com hooks

61d5990

for more information, see https://pre-commit.ci

Fixed Tutorial

f96223a

Merge branch 'JSONserialisation' of https://github.com/bennthomsen/br…

19b978b

…oadbean into JSONserialisation

Inlined Lambda function

39c1b1a

Merge branch 'main' into JSONserialisation

93d64c2

astafan8 reviewed Feb 3, 2026

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Added JSON serialisation for Lambda functions#1695

Added JSON serialisation for Lambda functions#1695
bennthomsen wants to merge 6 commits intoQCoDeS:mainfrom
bennthomsen:JSONserialisation

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bennthomsen commented Feb 2, 2026

Pull Request Summary

Title: Added JSON serialization for Lambda functions

Description

Changes

Files Modified:

Key Features

Usage Example

Technical Details

Testing

Breaking Changes

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