From concept to basic implementation

Building this code is kind like swallowing an elephant. There was no way I would just sit down and write it from one end to the other. So the first prototype of this did not have all the features it has now.

Spoiler warning, these next couple of sections will contain some Python typing details. When reading this, it might be helpful to know things such as Union[str, List[str]] being the Python type for either a str (string) or a List[str] (list of strings). If typing makes your head spin, these sections might less interesting for you.

To build this required a lot of playing around with Python's introspection and typing APIs. My very first draft only had one "schema" (the normalized form) and had the following features:

• Read TypedDict.__required_attributes__ and TypedDict.__optional_attributes__ to determine which attributes where present and which were required. This was used for reporting errors when the input did not match.
• Read the types of the provided TypedDict, strip the Required / NotRequired markers and use basic isinstance checks based on the resulting type for str and List[str]. Again, used for reporting errors when the input did not match.

This prototype did not take a long (I remember it being within a day) and worked surprisingly well though with some poor error messages here and there. Now came the first challenge, adding the manifest format schema plus relevant normalization rules. The very first normalization I did was transforming into: Union[str, List[str]] into into: List[str]. At that time, source was not a separate attribute. Instead, sources was a Union[str, List[str]], so it was the only normalization I needed for all my use-cases at the time.

There are two problems when writing a normalization. First is determining what the "source" type is, what the target type is and how they relate. The second is providing a runtime rule for normalizing from the manifest format into the target format. Keeping it simple, the runtime normalizer for Union[str, List[str]] -> List[str] was written as:

def normalize_into_list(x: Union[str, List[str]]) -> List[str]:
    return x if isinstance(x, list) else [x]

This basic form basically works for all types (assuming none of the types will have List[List[...]]). The logic for determining when this rule is applicable is slightly more involved. My current code is about 100 lines of Python code that would probably lose most of the casual readers. For the interested, you are looking for _union_narrowing in declarative_parser.py

With this, when the manifest format had Union[str, List[str]] and the target format had List[str] the generated parser would silently map a string into a list of strings for the plugin provider.

But with that in place, I had covered the basics of what I needed to get started. I was quite excited about this milestone of having my first keyword parsed without handwriting the parser logic (at the expense of writing a more generic parse-generator framework).

Adding the first parse hint

With the basic implementation done, I looked at what to do next. As mentioned, at the time sources in the manifest format was Union[str, List[str]] and I considered to split into a source: str and a sources: List[str] on the manifest side while keeping the normalized form as sources: List[str]. I ended up committing to this change and that meant I had to solve the problem getting my parser generator to understand the situation:

# Map from
class InstallExamplesManifestFormat(TypedDict):
    # Note that sources here is split into source (str) vs. sources (List[str])
    sources: NotRequired[List[str]]
    source: NotRequired[str]
    # We allow the user to write ``into: foo`` in addition to ``into: [foo]``
    into: Union[str, List[str]]

# ... into
class InstallExamplesTargetFormat(TypedDict):
    # Which source files to install (dest-dir is fixed)
    sources: List[str]
    # Which package(s) that should have these files installed.
    into: NotRequired[List[str]]

There are two related problems to solve here:

1. How will the parser generator understand that source should be normalized and then mapped into sources?
2. Once that is solved, the parser generator has to understand that while source and sources are declared as NotRequired, they are part of a exactly one of rule (since sources in the target form is Required). This mainly came down to extra book keeping and an extra layer of validation once the previous step is solved.

While working on all of this type introspection for Python, I had noted the Annotated[X, ...] type. It is basically a fake type that enables you to attach metadata into the type system. A very random example:

# For all intents and purposes, ``foo`` is a string despite all the ``Annotated`` stuff.
foo: Annotated[str, "hello world"] = "my string here"

The exciting thing is that you can put arbitrary details into the type field and read it out again in your introspection code. Which meant, I could add "parse hints" into the type. Some "quick" prototyping later (a day or so), I got the following to work:

# Map from
#     {
#        "source": "foo"  # (or "sources": ["foo"])
#        "into": "pkg"
#     }
class InstallExamplesManifestFormat(TypedDict):
    # Note that sources here is split into source (str) vs. sources (List[str])
    sources: NotRequired[List[str]]
    source: NotRequired[
        Annotated[
            str,
            DebputyParseHint.target_attribute("sources")
        ]
    ]
    # We allow the user to write ``into: foo`` in addition to ``into: [foo]``
    into: Union[str, List[str]]

# ... into
#     {
#        "source": ["foo"]
#        "into": ["pkg"]
#     }
class InstallExamplesTargetFormat(TypedDict):
    # Which source files to install (dest-dir is fixed)
    sources: List[str]
    # Which package(s) that should have these files installed.
    into: NotRequired[List[str]]

Without me (as a plugin provider) writing a line of code, I can have debputy rename or "merge" attributes from the manifest form into the normalized form. Obviously, this required me (as the debputy maintainer) to write a lot code so other me and future plugin providers did not have to write it.

High level typing

At this point, basic normalization between one mapping to another mapping form worked. But one thing irked me with these install rules. The into was a list of strings when the parser handed them over to me. However, I needed to map them to the actual BinaryPackage (for technical reasons). While I felt I was careful with my manual mapping, I knew this was exactly the kind of case where a busy programmer would skip the "is this a known package name" check and some user would typo their package resulting in an opaque KeyError: foo.

Side note: "Some user" was me today and I was super glad to see debputy tell me that I had typoed a package name (I would have been more happy if I had remembered to use debputy check-manifest, so I did not have to wait through the upstream part of the build that happened before debhelper passed control to debputy...)

I thought adding this feature would be simple enough. It basically needs two things:

1. Conversion table where the parser generator can tell that BinaryPackage requires an input of str and a callback to map from str to BinaryPackage. (That is probably lie. I think the conversion table came later, but honestly I do remember and I am not digging into the git history for this one)
2. At runtime, said callback needed access to the list of known packages, so it could resolve the provided string.

It was not super difficult given the existing infrastructure, but it did take some hours of coding and debugging. Additionally, I added a parse hint to support making the into conditional based on whether it was a single binary package. With this done, you could now write something like:

# Map from
class InstallExamplesManifestFormat(TypedDict):
    # Note that sources here is split into source (str) vs. sources (List[str])
    sources: NotRequired[List[str]]
    source: NotRequired[
        Annotated[
            str,
            DebputyParseHint.target_attribute("sources")
        ]
    ]
    # We allow the user to write ``into: foo`` in addition to ``into: [foo]``
    into: Union[BinaryPackage, List[BinaryPackage]]

# ... into
class InstallExamplesTargetFormat(TypedDict):
    # Which source files to install (dest-dir is fixed)
    sources: List[str]
    # Which package(s) that should have these files installed.
    into: NotRequired[
        Annotated[
            List[BinaryPackage],
            DebputyParseHint.required_when_multi_binary()
        ]
    ]

Code-wise, I still had to check for into being absent and providing a default for that case (that is still true in the current codebase - I will hopefully fix that eventually). But I now had less room for mistakes and a standardized error message when you misspell the package name, which was a plus.

The added side-effect - Introspection

A lovely side-effect of all the parsing logic being provided to debputy in a declarative form was that the generated parser snippets had fields containing all expected attributes with their types, which attributes were required, etc. This meant that adding an introspection feature where you can ask debputy "What does an install rule look like?" was quite easy. The code base already knew all of this, so the "hard" part was resolving the input the to concrete rule and then rendering it to the user.

I added this feature recently along with the ability to provide online documentation for parser rules. I covered that in more details in my blog post Providing online reference documentation for debputy in case you are interested. :)

Wrapping it up

This was a short insight into how debputy parses your input. With this declarative technique:

• The parser engine handles most of the error reporting meaning users get most of the errors in a standard format without the plugin provider having to spend any effort on it. There will be some effort in more complex cases. But the common cases are done for you.
• It is easy to provide flexibility to users while avoiding having to write code to normalize the user input into a simplified programmer oriented format.
• The parser handles mapping from basic types into higher forms for you. These days, we have high level types like FileSystemMode (either an octal or a symbolic mode), different kind of file system matches depending on whether globs should be performed, etc. These types includes their own validation and parsing rules that debputy handles for you.
• Introspection and support for providing online reference documentation. Also, debputy checks that the provided attribute documentation covers all the attributes in the manifest form. If you add a new attribute, debputy will remind you if you forget to document it as well. :)

In this way everybody wins. Yes, writing this parser generator code was more enjoyable than writing the ad-hoc manual parsers it replaced. :)