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Giving mypy a go

Introduction #

This is a follow up to the I don’t need types article.

I left a teaser explaining I’ll be giving concrete examples using a Python project, so here goes.

As you may have guessed, I’m going to talk about type annotations and the mypy static type checker.

Update: 2021-11-15: Feel free to also read the case study about urllib3

How mypy works #

Here’s a quick overview of how mypy works, in case you are not already familiar with it.

Let’s use a rather contrived example:

def is_odd(num):
    return num % 2 == 0

if is_odd("this sentence contains %s"):

We have a function is_odd, which obviously only works with numbers, and we call it with a string.

But since the string contains %s, Python will happily assume we are trying to format it, and the bug will go unnoticed. is_odd will simply return False, because we are also allowed to compare strings and numbers with == in Python.

Without type annotations, mypy detects nothing:

$ mypy

But if we add type annotations and re-run mypy, we do get an error message:

def is_odd(num: int) -> bool:
    return num % 2 == 0

if is_odd("this sentence contains %s"):
$ mypy
error: Argument 1 to "is_odd" has incompatible type "str"; expected "int"

Thus, you can use mypy in a loop:

  • Start by annotating a few functions.
  • Run mypy on your source code.
  • If it detects some errors, either fix the annotations or the code.
  • Back to step one

This is called “gradual typing”.

Designing an experiment #

The goal of the experiment is to check if going through the trouble of adding type annotations is worth it.

Some definitions #

Let me define what I mean by “worth it”.

There are many things said about types annotations like:

  • It makes it easier for external contributors to understand the code
  • It helps during refactorings
  • It facilitates maintenance of large projects

Those may be true, but they are hard to measure.

So I asked myself something else:

  • One: even with everything else (linters, tests, reviews …), were there bugs that only type annotations would catch?
  • Two: were the changes required to have mypy run without errors improving the quality of the code?

I know the second question is a bit subjective. I’m still going to use this metric because it’s one that really matters to me.

Choosing a project #

To test this hypothesis, I needed an existing project. I used tsrc, the command-line tool we use at work to manage multiple git repositories and automate GitHub and GitLab review automation.1

I chose it because:

  • It’s not too big, so the experiment will not take too long.
  • It’s not too small, so we’ll have enough experimental data.

The protocol #

So here is the protocol I used:

  • Go through the gradual typing loop until everything is type-annotated.
  • Make a note of every non-trivial patch. (That is anything that is not just adding annotations)
  • When the loop is finished, take a look at each of the patch, and ask if a bug was found, and whether it improved the quality of the code.

Before I continue, I should tell you I used mypy with three important options:

  • --strict: which means mypy emits an error for every missing type annotations.
  • --ignore-missing-imports: tsrc depends on libraries for which no type stub exist.
  • --strict-optional: If you have a string that can None, you must use Optional[str] instead of just str. I chose that because:
    • Errors relative to None are quite frequent
    • --strict-optional is going to be the default in a future mypy release.

Looking at patches #

Let’s look at trivial patches first:

Truthy string #

def find_workspace_path() -> Path:
    head = os.getcwd()
     tail = True
     while tail:
         tsrc_path = os.path.join(head, ".tsrc")
         if os.path.isdir(tsrc_path):
             return Path(head)
             head, tail = os.path.split(head)
     raise tsrc.Error("Could not find current workspace")

This function starts by checking there is a .tsrc hidden directory in the working path. If not, it goes through every parent directory and check if the .tsrc directory is here. If it reaches the root of the file system (the second value of os.path.split is None), it raises an exception.

mypy did not like that tail started as a boolean, and then was assigned to a string.

We can fix that by using a non-empty string, with a value that reveals the intention:

- tail = True
+ tail = "a truthy string"
     while tail:

An other way would be to use a type like Union[bool,str] but that would be more confusing I think.

Anyway, I’m not sure the quality of the code improved there. No point for mypy.

save_config #

class Options:
    def __init__(self, url, shallow=False) -> None:
        self.shallow: bool = shallow
        self.url: str = url

def save_config(options: Options):
    config = dict()
    config["url"] = options.url
    config["shallow"] = options.shallow

    with"w") as fp:
        ruamel.yaml.dump(config, fp)

We are using save_config to serialize a “value object” (the Options class) into a YAML file.

mypy saw the first two lines, config = dict(), config["url"] = options.ul and wrongly deduced that config was a dict from strings to strings.

Then it complained about config["shallow"] that was assigned to a boolean.

We can fix that by forcing the config type to be a dict from string to Any: Any is a “magic” type we can use precisely for this kind of situation.

- config = dict()
+ config: Dict[str, Any] = dict()

This a bit annoying, but the type annotation makes it clearer what the config is. 1 point for mypy.

Encoding project names #

When you use the GitLab API, you often have to use the ‘project id’. The doc says you can use the <namespace>/<project_name> string if it is “url encoded”, like this:

Get info about the Foo project in the FooFighters namespace:
GET  /api/v4/projects/FooFighters%2Foo

In python, the naive approach does not work:

import urllib.parse

>>> urllib.parse.quote("FooFighters/Foo")

Instead you have to specify a list of “safe” characters, that is characters that you don’t need to encode.

The doc says the default value is “/”.

Thus, here’s what we use in tsrc.gitlab:

project_id = "%s/%s" % (namespace, project)
encoded_project_id = urllib.parse.quote(project_name, safe=list())

mypy saw that the default value was a string, so he complained we were using a list instead. We fixed it by:

-        encoded_project_name = urllib.parse.quote(project_name, safe=list())
+        encoded_project_name = urllib.parse.quote(project_name, safe="")

Here, mypy forced us to follow an implicit convention. There are two ways to represent a list of characters in Python. A real list: ['a', 'b', 'c'], or a string: "abc". The authors of the urllib.quote() function decided to use the second form, so it’s a good thing we follow this convention too.

An other win for mypy.

Bugs found #

We already use a lot of tools in the hope of catching bugs:

  • Every pull request was reviewed by other humans
  • Two static analyzers (pylint and pyflakes) were ran for each pull request
  • McCabe complexity was measured for each and every function and method of the code base and was not allowed to go above 10.
  • TDD was used throughout the development of the project
  • Test coverage was already at 80%

Despite of this, mypy found bugs that tests, all the linters and all the reviewers did not find.

Here are a few of them. Feel free to try and find the bug yourself, the answer will be given after the ⁂ symbol.

handle_stream_errors #

class GitLabAPIError(GitLabError):
    def __init__(self, url: str, status_code: int, message: str) -> None:

def handle_stream_errors(response: requests.models.Response) -> None:
     if response.status_code >= 400:
        raise GitLabAPIError(
            response.url, "Incorrect status code:", response.status_code)

This code wraps calls to the GitLab API, and arrange for a particular GitLabAPIError to be raised, when we make a “stream” request. (For instance, to download a GitLab CI artifact):

The problem here is that we inverted the status_code and message parameter. Easy to fix:

-        raise GitLabAPIError(
-           response.url, "Incorrect status code:", response.status_code)
+        raise GitLabAPIError(
+           response.url, response.status_code, "Incorrect status code")

The bug was not caught because the code in question was actually copy/pasted from a CI script (and you usually don’t write tests for CI scripts).

We actually don’t need streamed responses anywhere in tsrc, so this is in fact dead code (and it is now gone from the code base)

handle_json_errors #

def handle_json_errors(response: requests.models.Response):

         json_details = response.json()
     except ValueError:
     json_details["error"] = (
         "Expecting json result, got %s" % response.text


    url = response.url
    if 400 <= status_code < 500:
        for key in ["error", "message"]:
            if key in json_details:
                raise GitLabAPIError(url, status_code, json_details[key])
         raise GitLabAPIError(url, status_code, json_details)
    if status_code >= 500:
        raise GitLabAPIError(url, status_code, response.text)

This one is slightly more interesting. It is located near the previous one and handles errors for the calls to the GitLab API which returns JSON objects.

We of course have to catch 500 errors, which hopefully do not happen often.

In case of a status code between 400 and 499, we know there was a problem in the request we made, but we need to tell the user why the request was rejected.

Most of the time the GitLab API returns a JSON object containing a error or message key, but sometimes neither key is found in the returned object, and sometimes the text of the response is not even valid JSON code.

So we have to check for both keys in the JSON object, and if not found (if we exit the for loop), just store the entire JSON response in the exception.

The bug is in the second raise GitLabAPIError. We are passing an entire object where the GitLabAPIError expected a string.

The fix was:

- raise GitLabAPIError(url, status_code, json_details)
+ raise GitLabAPIError(url, status_code, json.dumps(json_details, indent=2))

Again, this was hard to catch with tests. The case where the JSON returned by GitLab did not contain a error or message key only happened once in the lifetime of the project (which explain why the code was written), so manual QA and unit tests did not need to check this code path.

Anyway, note we did not blindly wrote something like str(json_details) to convert the JSON object to a string. We found out it was used in a message displayed to the end user, thus we use json.dumps(json_details), indent=2) to make sure the message contains neatly indented JSON and is easy to read.

LocalManifest #

class LocalManifest:
    def __init__(self, workspace_path: Path) -> None:
        hidden_path = workspace_path.joinpath(".tsrc")
        self.clone_path = hidden_path.joinpath("manifest")
        self.cfg_path = hidden_path.joinpath("manifest.yml")
        self.manifest: Optional[tsrc.manifest.Manifest] = None

    def update(self) -> None:
        ui.info_2("Updating manifest")
        tsrc.git.run_git(self.clone_path, "fetch")
        tsrc.git.run_git(self.clone_path, "reset", "--hard", branch)

    def load(self) -> None:
        yml_path = self.clone_path.joinpath("manifest.yml")
        if not yml_path.exists():
            message = "No manifest found in {}. Did you run `tsrc init` ?"
            raise tsrc.Error(message.format(yml_path))
        self.manifest = tsrc.manifest.load(yml_path)

    def copyfiles(self) -> List[Tuple[str, str]]:
        return self.manifest.copyfiles

    def get_repos(self) -> List[tsrc.Repo]:
        assert self.manifest, "manifest is empty. Did you call load()?"
        return self.manifest.get_repos(groups=self.active_groups)

After you run tsrc init, the manifest is cloned inside <workspace>/.tsrc/manifest.

Thus, the contents of the manifest.yml is in <workspace>/.tsrc/manifest/manifest.yml>.

The LocalManifest class represent this manifest repository.

Here’s what happens when you run tsrc sync:

  • local_manifest.update() is called: the repository in <workspace>/.tsrc/manifest> is updated by running git fetch; git reset --hard origin/master
  • local_manifest.load() is called: the manifest.yml file is parsed, and its contents are stored in the self.manifest attribute.
  • Then the Syncer class calls local_manifest.get_repos() to find out the list of repositories to clone or synchronise.
  • Finally, the FileCopier class uses local_manifest.copyfiles to perform file copies.

It’s not really a bug, but mypy forced us to acknowledge that LocalManifest.manifest starts as None, and only gets its real value after .load() has been called.

We already have an assert in place in get_repos(), but mypy forced us to add a similar check in the copyfiles getter:

    def copyfiles(self) -> List[Tuple[str, str]]:
+       assert self.manifest, "manifest is empty. Did you call load()?"
        return self.manifest.copyfiles

GitServer #

Some of the tests for tsrc are what we call end-to-end tests:

Here’s how they work:

  • We create a band new temporary directory for each test
  • In it, we create a srv directory with bare git repositories
  • Then, we create a work directory and we run tsrc commands from there.

Thus, we don’t have to mock file systems or git commands (which is doable but pretty hard), and things are easy to debug because in case of problem we can just cd to the test directory and inspect the state of the git repositories by hand.

Any way, those tests are written with a test helper called GitServer.

You can use this class to create git repositories, push files on some branches, and so on:

Here’s what the helper looks like:

    def add_repo(self, name: str, default_branch="master") -> str:
        url = self.get_url(name)
        return url

    def push_file(self, name: str, file_path: str, *,
                  contents="", message="") -> None:
        full_path = ...
        if contents:
        commit_message = message or ("Create/Update %s" % file_path)
        run_git("add", file_path)
        run_git("commit", "--message", commit_message)
        run_git("push", "origin", "--set-upstream", branch)

    def tag(self, name: str, tag_name: str) -> None:
        run_git("tag", tag_name)
        run_git("push", "origin", tag_name)

    def get_tags(self, name: str) -> List[str]:
         src_path = self.get_path(name)
         rc, out = tsrc.git.run_git_captured(src_path, "tag", "--list")
         return out

You can use it like this:

def test_tsrc_sync(tsrc_cli: CLI, git_server: GitServer) -> None:
    manifest_url = git_server.manifest_url"init", manifest_url)
    git_server.push_file("foo/bar", "bar.txt", contents="this is bar")"sync")

    bar_txt_path = workspace_path.joinpath("foo", "bar", "bar.txt")
    assert bar_txt_path.text() == "this is bar"

The bug is in get_tags:

    def get_tags(self):
         rc, out = tsrc.git.run_git(src_path, "tag", raises=False)
-        return out
+        return out.splitlines()

The get_tags methods is also dead code. It has an interesting story.

The need for the GitServer helper occurred when I started writing end-to-end tests for tsrc and discovered I needed a test framework to write those end-to-end test.

It was vital that the GitServer implementation uses clean code.

  • One, we must be sure GitServer has no bugs. Otherwise, we may get stuck looking for bugs in our production code when tests fail.
  • For the same reason, GitServer should be easy to change. There will almost certainly be features added in tsrc that will require adapting GitServer code in order to properly test it.

So to have a clean implementation of the GitServer I of course used the best technique I know of: TDD.

yo dawg tests

You can find them in the []( file.

Anyway, I was writing an end-to-end test for tsrc that involved tags.

I thought: “OK, I need a .tag() method in GitServer. So I also need a get_tags() method to check the tag was actually pushed”. Thus I wrote the get_tags method, forgetting to write a failing test for GitServer first (that still happens after 5 years of TDD, so don’t worry if it happens to you too.). At that point I only had my end-to-end test failing, so I made it pass and completely forgot about the get_tags() method. Oh well.

Executors and Tasks #

In the implementation of tsrc we often have to loop over a list of items, perform actions an each of them and record which one failed, and when the loop is done, display the error messages to the user.

For instance:

tsrc sync
* (1/2) foo/bar
remote: Counting objects: 3, done.
Updating 62a5d28..39eb3bd
error: The following untracked files would be overwritten by merge:
Please move or remove them before you merge.
* (2/2) spam/eggs
Already up to date.
Error: Synchronize workspace failed
* foo/bar: updating branch failed


$tsrc foreach
:: Running `ls foo` on every repo
* (1/2) foo
$ ls foo
* (2/2) spam
$ ls foo
ls: cannot access 'foo': No such file or directory
Error: Running `ls foo` on every repo failed
* spam

So in order to keep things DRY, we have some high-level code that only deals with loop and error handling:

First, a generic Task method.

(Note that you can have your own generic types with mypy. This is awesome because without it you get laughed at by C++ programmers)

class Task(Generic[T], metaclass=abc.ABCMeta):

    def description(self) -> str:

    def display_item(self, item: T) -> str:

    def process(self, item: T) -> None:

And then, a generic SequentialExecutor who knows how to execute a given task on a list of items and display the outcome:

class SequentialExecutor(Generic[T]):
    def __init__(self, task: Task[T]) -> None:
        self.task = task
        self.errors: List[Tuple[Any, tsrc.Error]] = list()

    def process(self, items: List[T]) -> None:
        if not items:
            return True

        self.errors = list()
        for item in enumerate(items):

        if self.errors:

    def process_one(self, item: T) -> None:
        except tsrc.Error as error:
            self.errors.append((item, error))

    def handle_errors(self) -> None:
        # Display nice error message
        raise ExecutorFailed()

Thus we can inherit from Task to implement tsrc sync:

class Syncer(tsrc.executor.Task[Repo]):

    def process(self, repo: Repo) -> None:

    def check_branch(self, repo: Repo):
        current_branch = tsrc.git.get_current_branch(repo_path)
        if not current_branch:
            raise tsrc.Error("Not on any branch")

Ditto for tsrc foreach:

class CmdRunner(tsrc.executor.Task[Repo]):

    def process(self, repo: Repo) -> None:
        full_path = self.workspace.joinpath(repo.src)
        rc =, ...)
        if rc != 0:
            raise CommandFailed

Did you spot the bug?

The bug is in here:

    def process(self, items: List[T]) -> None:
         if not items:
             return True

This is the result of a bad refactoring. Executor used to track success of task by looking at the return value the process() method.

After a while, I found out it was clearer to just use exceptions for that, mostly when I implemented the Syncer class. (You can just raise an exception instead of adding lots of ifs).

But the early return True was left. Here mypy found something that would have puzzled an external reader. Almost everything function or method dealing with Executors and Tasks in tsrc either return None or raise Exception. What is that boolean doing here?

There were of course no tests left that checked the return value of process (they got refactored at the same time of the rest of the code), so the bug went unnoticed.

run_git #

The last changed required by mypy was also pretty interesting. Here’s the deal.

In tsrc, we often need to run git commands, but we can run them in two very different ways:

  • We can either just run the command and let its output directly being displayed to the user. This is useful for things like git fetch which can take some times and for which the output contains an indication of progress for the user.
  • Or we just need to run the command, capture its output and deal with it as a value. For instance, if we want to get the url of the “origin” remote, we can call git remote get-url origin.
  • In both cases, we must handle the possibility that the command can fail.

Here’s what the implementation looked like:

def run_git(working_path, *cmd, raises=True):
    """ Run git `cmd` in given `working_path`

    If `raises` is True and git return code is non zero, raise
    an exception. Otherwise, return a tuple (returncode, out)

    git_cmd = list(cmd)
    git_cmd.insert(0, "git")
    options = dict()
    if not raises:
        options["stdout"] = subprocess.PIPE
        options["stderr"] = subprocess.STDOUT

    process = subprocess.Popen(git_cmd, cwd=working_path, **options)

    if raises:
        out, _ = process.communicate()
        out = out.decode("utf-8")

    returncode = process.returncode
    if raises:
        if returncode != 0:
            raise GitCommandError(working_path, cmd)
        return returncode, out

And here’s how to use it:

# run `git fetch`, leaving the output as-is
run_git(foo_path, "fetch")

# run git remote get-url origin
rc, out = run_git(foo_path, "remote", "get-url", "origin", raises=False):
if rc == 0:
    # Handle the case where there is no remote called 'origin'
    # Do something with out

Here’s an other place we used run_git: we have a command named tsrc status, which displays a summary of the whole workspace. Here what’s its output looks like:

tsrc status
* foo master ↓1 commit
* bar devel ↑1 commit

And here is the implementation:

class GitStatus:

    def update_remote_status(self):
        _, ahead_rev = run_git(
            "rev-list", "@{upstream}..HEAD",
        self.ahead = len(ahead_rev.splitlines())

        _, behind_rev = run_git(
            "rev-list", "HEAD..@{upstream}",
        self.behind = len(behind_rev.splitlines())

Found the bug yet?

The code in update_remote_status() assumes that git rev-list outputs a list of commits, and then count the lines in the output.

This works well if there is a remote branch configured:

$ git revlist HEAD..@{upstream}

But if not remote is configured, the git command will fail with a message looking like:

$ git revlist HEAD..@{upstream}
fatal: no upstream configured for branch 'dm/foo'

Since update_remote_status() does not check the return code, self.ahead_rev and self.behind_rev both get set to 1, and the output looks like:

* foo other-branch ↑1↓1 commit


But there is more to it than just this bug.

Notice the type of the return value of run_git depends on the value of the raises parameter.

At first I tried to annotate the function with a Union type, like this:

def run_git(working_path: Path, *cmd: str, raises=True) ->
      Union[Tuple[int, str], None]:

But then mypy complained for each and every line that used run_git with raises=False:

rc, out = run_git(..., raises=False) error: 'builtins.None' object is not iterable

I thought about this and found out it was cleaner to split run_git into run_git and run_git_captured:

def run_git(working_path: Path, *cmd: str) -> None:
    """ Run git `cmd` in given `working_path`

    Raise GitCommandError if return code is non-zero.
    git_cmd = list(cmd)
    git_cmd.insert(0, "git")

    returncode =, cwd=working_path)
    if returncode != 0:
        raise GitCommandError(working_path, cmd)

def run_git_captured(working_path: Path, *cmd: str,
                     check: bool = True) -> Tuple[int, str]:
    """ Run git `cmd` in given `working_path`, capturing the output

    Return a tuple (returncode, output).

    Raise GitCommandError if return code is non-zero and check is True
    git_cmd = list(cmd)
    git_cmd.insert(0, "git")
    options: Dict[str, Any] = dict()
    options["stdout"] = subprocess.PIPE
    options["stderr"] = subprocess.STDOUT

    returncode = process.returncode
    if check and returncode != 0:
        raise GitCommandError(working_path, cmd, output=out)
    return returncode, out

True, there’s a bit more code and slightly duplication but:

  • The multiple if raises in the original implementation are gone. Less if in a function is always a win.
  • The raises parameter did too very different things:
    • One, it changed the return type of the function
    • Two, it forced the caller to explicitly handle the case where the command fails.

Now the intent about whether you want to capture the output or not is encoded in the name of the function (run_git_captured instead of run_git).

Also note that you can’t forget about checking the return code anymore.

You can either write:

_, out = run_git_captured(repo_path, "some-cmd")
# We know `out` is *not* an error message, because if `some-cmd` failed, an
# exception would have been raised before `out` was set.

Or you can be explicit about your error handling:

rc, out = run_git_captured(repo_path, "some-cmd", check=False)
if rc == 0:
    # Ditto, out can't be an error message
    # Need to handle error here

That alone would be a good reason to use mypy I think :)

What’s next #

Well, the mypy changes have been merged and the CI now runs mypy in strict mode on every pull request.

I’m still curious about the other benefits of type annotations I could not check (maintainability, code comprehension, ease of refactorings …).

I guess we’ll see how the next refactoring in tsrc goes.

Update, a few months later: Well, the next refactoring in tsrc went really well and mypy caught a ton of problems very early. I’m now using mypy in strict mode in almost all of my projects.

Conclusion #

We saw how mypy, while stilling making relative few false positives, still found inconsistencies, a few bugs, and even design problems.

So, my advice is for you to use it if you get a chance. Cheers!

  1. If you want to know more, feel free to browse the documentation, or read the introduction post↩︎

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