Project: Tryceratops ๐Ÿฆ–โœจ

gui commited 2 years ago · ๐Ÿ Python ·
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GitHub - guilatrova/tryceratops: A linter to prevent exception handling antipatterns in Python (limited only for those who like dinosaurs).
A linter to prevent exception handling antipatterns in Python (limited only for those who like dinosaurs). - GitHub - guilatrova/tryceratops: A linter to prevent exception handling antipatterns in ...

Stack: Python

It's a linter to help python developers write nice and clean try / except blocks.

Why a linter?

I got very interested in writing a linter, mostly because I had no idea how I would be able to use Python to lint more Python.

It seemed so hard that I wondered if I would be able to do it. More important than that, I realized that:

๐Ÿ“ˆ People seemed interested in writing better exceptions in Python

After publishing my article on Handling exceptions in Python like a pro ๐Ÿ ๐Ÿ’ฃ I got so surprised by the increased traffic in this blog:

It went from ~1-2 people daily (me and my mom probably) to ~20-50 people daily.

traffic today

So it made me think that writing good and well-defined try/except blocks would be something that developers would like to do.

๐Ÿฐ Linters make your life easier

Without linters, would you be able to manage every unused import in your project? Maybe, but it would be a pain and it's very easy to forget about it.

That's another reason that made me realize that having a tool to check my code and give me feedback is a great way to keep the quality without the struggle.

๐Ÿ Processing Python Code

The first question is: How can Python understand Python? I started considering reading a file like a bare string (e.g. def func(): return 1), splitting it by whitespaces, and reading it token by token.

No, I didn't do that.

Thank God it was not required. Python has ast that made it insanely simple.

๐ŸŒณ Interpreting code with AST

AST stands for Abstract Syntax Trees which means you can read Python code, any block you wish, iterate over the syntax, and process it. Combine this with astpretty and you have a powerful debugging tool.

My process developing this linter was:

  1. Creating some sample Python code with a violation I would like to help with;
  2. Running astpretty on the sample file to understand what I wanted to check;
  3. Coding the analyzer logic;
# Output from: astpretty --no
            value=Constant(value="\nViolation:\n\nDon't use bare except\n", kind=None),
            args=arguments(posonlyargs=[], args=[], vararg=None, kwonlyargs=[], kw_defaults=[], kwarg=None, defaults=[]),
                            targets=[Name(id='a', ctx=Store())],
                            value=Constant(value=1, kind=None),
                            body=[Raise(exc=None, cause=None)],

As you can see, the top-level object is the Module which contains a body with Expr and FunctionDef, which also has a body and so on...

The effort to achieve that is nearly 0, your only job is to read the python file as text and invoke ast.parse.

๐Ÿšช Visiting Nodes

I relied on ast.NodeVisitor class to analyze my code. It allows me to iterate and filter over nodes (e.g. FunctionDef). The behavior is simple, straightforward, and helps to dive into specific statements when they happen.

You just have to inherit from that class, define a method named visit_[NODE_NAME](self, node) and we're good to start coding.

Take for example the analyzer to avoid verbose reraises.

The goal is to avoid people from writing:

except Exception as ex:
    raise ex  # <-- This is verbose

So they can write it like:

except Exception:
    raise  # <-- This is clean

With astpretty help I learned that I want to visit ExceptHandler objects that contains any name, and iterate from there to find violations. The final analyzer code is::

class ExceptVerboseReraiseAnalyzer(BaseAnalyzer, ast.NodeVisitor):  # <-- Inherit from ast.NodeVisitor
    violation_code = codes.VERBOSE_RERAISE

    def visit_ExceptHandler(self, node: ast.ExceptHandler) -> None:  # <-- We only want "except" blocks
        def is_raise_with_name(stm: ast.stmt, name: str):
            if isinstance(stm, ast.Raise) and isinstance(stm.exc, ast.Name):
                return == name
            return False

        # If no name is set, then it's impossible to be verbose
        # since you don't have the object
            for child in ast.walk(node):  # <-- We iterate over all children in that node
                if is_raise_with_name(child,  # <-- If condition is true, we found a violation


๐Ÿ’ฌ Capturing Comments in Python with tokenize

I never desired to enforce all linter rules on the developer. I like flexibility and, well, the code is yours. I want you to disable rules that don't please you by settings comments like: # notc: VIOLATION CODE.

After gathering all violations in code using ast, I thought it would be very easy to capture such comments.

Turns out that I was wrong...

ast does not handle comments, after all, comments are not python code.

I had then to use another lib (tokenize) to do that. This was boring because I had to read line by line and filter any comments that matches a regex, see the function:

def parse_ignore_comments(content: TextIOWrapper) -> Generator[IgnoreViolation, None, None]:
    for toktype, tokval, start, *_ in tokenize.generate_tokens(content.readline):
        if toktype == tokenize.COMMENT:
            if match :=, tokval):
                yield _build_ignore_line(match, start)

Then Tryceratops CLI ended up handling a tuple: (filename, ast, ignored-comments).

๐Ÿšฆ Testing scenarios with Pytest

Another cool challenge was: how to write tests? How can someone keep track of python code to be tested under python code?

For that, there's a directory tests/samples for python files that intentionally violates specific rules.

The testing got very simple once I defined 2 helper functions: read_sample and assert_violation

def read_sample(filename: str) -> ast.AST:
    ref_dir = f"{os.path.dirname(__file__)}/samples/violations/"
    path = f"{ref_dir}{filename}.py"

    with open(path) as sample:
        content =
        loaded = ast.parse(content)
        return loaded

def assert_violation(code: str, msg: str, line: int, col: int, violation: Violation):
    assert violation.line == line
    assert violation.col == col
    assert violation.code == code
    assert violation.description == msg

Testing violations got very simple, I don't even need to mock anything.

def test_verbose_reraise():
    tree = read_sample("except_verbose_reraise")
    analyzer = analyzers.ExceptVerboseReraiseAnalyzer()
    code, msg = codes.VERBOSE_RERAISE

    assert_verbose = partial(assert_violation, code, msg)  # <-- I'm lazy, make function shorter

    violations = analyzer.check(tree, "filename")  # <-- Run analyzer
    assert len(violations) == 2
    first, second = violations

    assert_verbose(20, 8, first)  # <-- assert line, offset, expected violation
    assert_verbose(28, 12, second)  # <-- assert line, offset, expected violation

After getting the first analyzer passing, I started following a TDD approach where I first create a file with the intended violation, then write the test, and finally implement the passing code.

It's hard to test such stuff manually, so writing unit tests made me a lot more productive.

๐Ÿงพ CLI and Configs

Once the analyzers were set (and somewhat working) I had to make it usable, and linters fit well as a command-line interface, so you can run it like: tryceratops or maybe tryceratops pydir.

Using click as Python CLI

Click is a simple framework to wrap your python code around a command-line interface quickly. It's as simple as wrapping your function with a few decorators, see:

EXPERIMENTAL_FLAG_OPTION = dict(is_flag=True, help="Whether to enable experimental analyzers.")
    help="A violation to be ignored. e.g. -i TC200 -i TC201",
EXCLUDE_OPTION = dict(multiple=True, help="A dir to be excluded. e.g. -x tests/ -x fixtures/")

@click.argument("dir", nargs=-1)
@click.option("--experimental", **EXPERIMENTAL_FLAG_OPTION)
@click.option("-i", "--ignore", **IGNORE_OPTION)
@click.option("-x", "--exclude", **EXCLUDE_OPTION)
def entrypoint(dir: Tuple[str], experimental: bool, ignore: Tuple[str, ...], exclude: Tuple[str, ...]):

def main():

if __name__ == "__main__":

Once I set some decorators like @click.option I had to receive and manage specific arguments which reflect the options I defined. It's indeed simple. The annoying part was to figure out how to use some options (the documentation is not clear).

It took me a while to find out:

Supporting pre-commit

I love pre-commit, it ensures that I'm not committing some sort of code that would be a problem on CI, that I'm using all assigned vars, that all imports are being used, and that some other small improvements like end-of-line and trailing whitespace are clean. I wanted to promote the same feature for devs to benefit from this as well.

It's indeed very simple, I just had to create a file named: pre-commit-hooks.yaml with the following:

-   id: tryceratops
    name: tryceratops
    description: "Manage your exceptions in Python like a PRO"
    entry: tryceratops
    language: python
    language_version: python3
    types: [python]
    require_serial: true

Easy! Now anyone can add a new pre-commit hook in their repo like:

  - repo:
    rev: v0.2.3
      - id: tryceratops

Supporting pyproject.toml

The next step would be to support PEP518 by allowing configs to be defined at the project level. Maybe you don't like some violation that I defined, that's fine, it shouldn't refrain you from benefiting from using Tryceratops.

The goal is to support configs that someone would usually pass through CLI, so this:

tryceratops dir -x samples -i TC002 -i TC200 -i TC300 --experimental

can be represented by this:

exclude = ["samples"]
ignore = ["TC002", "TC200", "TC300"]
experimental = true

Reading the file was quite easy, there's a lib named toml that, guess what, reads and generates toml files.

import toml


def load_config(dir: Sequence[str]) -> Optional[PyprojectConfig]:
    toml_file = find_pyproject_toml(dir)

    if toml_file:
        config = toml.load(toml_file)
        return config.get("tool", {}).get("tryceratops", {})

The challenge would be to find out whether a pyproject.toml file exists or not.

To be honest I wasn't sure on the best way to do it. Someone can run tryceratops from anywhere in the directory tree and it should always work! So I can't infer that the CLI is always being invoked from the project root.

Instead of struggling to find a solution myself (that probably would be not that good), I realized that I could learn what other projects are doing. For instance, I checked on black and adjusted it (by simplifying) to fit my needs, that's what I ended up with:

def find_project_root(srcs: Sequence[str]) -> Path:
    """Return a directory containing .git, .hg, or pyproject.toml.

    That directory will be a common parent of all files and directories
    passed in `srcs`.

    If no directory in the tree contains a marker that would specify it's the
    project root, the root of the file system is returned.
    if not srcs:
        srcs = [str(Path.cwd().resolve())]

    path_srcs = [Path(Path.cwd(), src).resolve() for src in srcs]

    # A list of lists of parents for each 'src'. 'src' is included as a
    # "parent" of itself if it is a directory
    src_parents = [list(path.parents) + ([path] if path.is_dir() else []) for path in path_srcs]

    common_base = max(
        set.intersection(*(set(parents) for parents in src_parents)),
        key=lambda path:,

    for directory in (common_base, *common_base.parents):
        if (directory / ".git").exists():
            return directory

        if (directory / ".hg").is_dir():
            return directory

        if (directory / "pyproject.toml").is_file():
            return directory

    return directory

def find_pyproject_toml(path_search_start: Tuple[str, ...]) -> Optional[str]:
    """Find the absolute filepath to a pyproject.toml if it exists"""
    path_project_root = find_project_root(path_search_start)
    path_pyproject_toml = path_project_root / "pyproject.toml"
    if path_pyproject_toml.is_file():
        return str(path_pyproject_toml)

It's a very great idea to check on .git or pyproject.toml itself to decide whether it's the project root and it worked just fine.

๐Ÿ“ฆ Publishing Package to Pypi with Flit

Publishing to pypi is the fun part! Making it available and easy distributable for everyone so a dev can simply pip install tryceratops is as magic โœจ as running dinosaur commands ๐Ÿฆ– from your terminal. There're several distinct ways of publishing a package to Pypi, and flit is the simplest one that I tried so far.

I didn't have to care about creating a and the docs seemed too boring in comparison to the simplicity of writing a config in pyproject.

The only challenge is to (remember to) keep requires up to date. I released two versions already without the proper dependencies set so they're unusable ๐Ÿ˜. Sadly Pypi doesn't allow me to overwrite older published versions with a new package.

๐Ÿ›ฃ Roadmap and Plans

I would say the next steps are:

๐Ÿ•ต๏ธโ€โ™€๏ธ Test it more and collect real user feedback

Is it really useful? Does it really work? Is there any hidden bug?

Such questions I can only find out by using the tool more and more.

It's hard to promote something that is still beta, hopefully, I'll figure this out as I keep working on it.

Please, let me know if you/your company found it useful, I'd be flattered to add your project to the main README as a "Tryceratops Supporter" ๐Ÿฆ–๐Ÿ† !

Critics and feedback are ALWAYS welcome, tell me if you hate it! I swear I won't cry ๐Ÿฅฒ.

๐Ÿš€ Implement CI/CD

So far the release process is very manual! It requires me to run flit, update readme with the new version, create a release on GitHub (for pre-commit), and I can't ensure tests are still passing.

I started the project following conventional commits which is the perfect match of semantic relase to automate versioning + generate changelogs + releases. I hope I can find the time to implement it someday later in the future.

๐Ÿšง Extend violations and analyzers with more scenarios

I need more scenarios! Yes, it's hard to create situations! For example, I realized that TC003 is raised for: raise Exception("long message") but not for raise module.Exception("long message"). I need to find edge cases and make this tool robust, otherwise, it fails by not providing the expected value.

Besides this, I believe there's still value in adding a few more violations for the logging usage within except blocks (which I don't think people usually know how to use properly, for example, do you know the difference between logging.error and logging.exception?).

I'm tracking planning and progress on the GitHub project so anyone can check out.


I must thank God for the inspiration ๐Ÿ™Œ , He is my source of creativity.

The black project for insights, inspiration, and especially for the pyproject.toml project root code.

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