How can I load Python lambda expressions from YAML files using ruamel.yaml?

Question

I'm trying to serialize and deserialize objects that contain lambda expressions using ruamel.yaml. As shown in the example, this yields a ConstructorError. How can this be done?

import sys
import ruamel.yaml

yaml = ruamel.yaml.YAML(typ='unsafe')
yaml.allow_unicode = True
yaml.default_flow_style = False

foo = lambda x: x * 2
yaml.dump({'foo': foo}, sys.stdout)
# foo: !!python/name:__main__.%3Clambda%3E

yaml.load('foo: !!python/name:__main__.%3Clambda%3E')
# ConstructorError: while constructing a Python object
# cannot find '<lambda>' in the module '__main__'
#   in "<unicode string>", line 1, column 6

Answer 1

That is not going to work. ruamel.yaml dumps functions (or methods) by making references to the those functions in the source code by referring to their names (i.e. it doesn't try to store the actual code).

Your lambda is an anonymous function, so there is no name that can be properly retrieved. In the same way Python's pickle doesn't support lambda.

I am not sure if it should be an error to try and dump lambda, or that a warning should be in place.

The simple solutions is to make your lambda(s) into named functions. Alternatively you might be able to get to the actual code or AST for the lambda and store and retrieve that, but that is going to be more work and might not be portable, depending on what you store.