First Experience with Mypy


I have a trello board with a to Learn column, and Mypy was featuring prominently.

For those who are not aware, Mypy is a static checker for Python. The idea is to annotate Python objects and functions with indications about what type an object can be (a string, an "integer or None", etc.). You can then run a checker on the codebase and it will warn you about wrong type usage. At runtime nothing change: Python is still Python, not another language, and the type annotations are just ignored.

def greeting(name: str) -> str:
    return 'Hello ' + name

It's like using a language less flexible than Python, but it still runs as slow as Python, because it's not compiled. Ehm... the worst of two world? 😬

So, why people do it? Well, people do weird things. But the idea behind it is that there is a certain class of bugs that Mypy checker would catch. The lack of type checking in Python is at the heart of its philosophy (duck typing and all that stuff). On the other hand many people, and I amongst them, can be nervous in putting in production code with little formal assurance about its correctness. My personal way to cope with the uneasiness of a language without compiler is to resort to write unit tests. Frameworks like pytest make writing and running tests a joy, reducing to almost zero the need of fiddling interactively into an interactive console while developing. I also find that writing tests before writing the code they are supposed to test helps in thinking what do you want your code to look like and to design more usable interfaces.

So, big proposer of TDD here, stressing on the second D because writing test while you develop can actually happen and is useful; writing "the program now, the tests later" can be dismissed with a laugh: I have never seen that happening.

As I started writing psycopg3, tests have been committed together with the code and, on a codebase of less than 3000 lines of code, there are already some 150 of them. They get written to help with development but they are also run at every commit on Travis CI, making sure that changes in the code don't introduce breakage in any of the supported versions of Python (from 3.6 to 3.8) and PostgreSQL (from 9.5 to 12).

While this is a strong position from which to say "who needs a compiler?" and to laugh smugly at developers over-relying on a compiler and tripping on java.lang.NullPointerExceptions I don't think being complacent is any useful for the quality of a codebase, or of a person for what matters. And if so much research is being done on the Python objects model it surely deserves a look.

Just running mypy on the whole codebase produces little of interest. Running mypy --strict is a different story, because it is expected every function to be annotated. I started at a 250 errors mark:

(env) piro@makkuro:~/dev/psycopg3$ mypy --strict psycopg3
psycopg3/ error: Function is missing a type annotation
psycopg3/ error: Function is missing a type annotation
[... skipping pain here]
psycopg3/types/ error: Call to untyped function "register" of "Typecaster" in typed context
psycopg3/types/ error: Function is missing a type annotation
Found 253 errors in 13 files (checked 20 source files)

Fast forward 6 hours of hacking, not solid but surely intense and:

(env) piro@makkuro:~/dev/psycopg3$ mypy --strict psycopg3
Success: no issues found in 21 source files

What did I learn from the experience?

The changes

In all its glory, this is the whole changeset to go from no type to a clean --strict pass.

It's a whopping 794 additions and 365 deletions, which is a relevant amount of changes for a codebase of less than 3000 lines. What was that for? Here are some of the changes I ended up making, and some observations about them.

Local and class variables

You don't have to annotate every single variable: the types are inferred automatically. But when things cannot be inferred from the first value assumed by a variable it is necessary to add a type. That's fair.

  def _reset(self) -> None:
-     self._results = []
-     self._result = None
+     self._results: List[PGresult] = []
+     self._result: Optional[PGresult] = None

Out-of-domain values

A type of change I wasn't especially keen on initially was in situations like:

ready = None

while 1:
    if something_happens():
        ready = Ready.R
        ready = Ready.W

    if we_have_done(ready):

Mypy would complain along the lines of:

Argument 1 to "we_have_done" has incompatible type "Optional[Ready]"; expected "Ready"

Uhm. In my book a None will never reach we_have_done(). However go tell the analyzer. In order to make the checker happy I changed the above to:

- ready = None
+ ready = Ready.R

which is not something that made me happy. It gives the wrong idea that the initial Ready.R state is actually meaningful, when all it means instead is "shut up, Mypy". Only later it occurred to me that a better fix is probably:

- ready = None
+ ready: Ready

meaning that the type is declared although it doesn't have a value yet. In case a later refactoring was botched, the code would explode here, with a NameError in this function, where the bug actually is, rather than somewhere in the depths of the innocent we_have_done(). So I think this is a positive change, bringing a possible manifestation of an error closer to its cause.

Too creative decorators?

Adaptation of Python objects to PostgreSQL types happens via adapters, for which I'm toying with various choices to make them easy to define, easier than what it is in psycopg2 and covering tricky corner cases more naturally. In one way or another, the need is to store in a registry the association from a Python type to an adapter object, let's say a function for simplicity.

In the form I had when I started introducing types, a static method Adapter.register() could be used in both ways: as a function, e.g.:

def adapt_number(s):

Adapter.register(int, adapt_number)

or as a decorator:

def adapt_number(s):

both are useful in different situations: the former is useful when defining or registering the adapter at import time is not convenient, the latter is a matter of convenience, as all decorators are.

But what's the price of this convenience? In this implementation the function has a signature of the likes of Adapt.register(type,obj=None), behaving in radically different ways according to whether obj is None or not. If it's not None (the function call case) then it registers the function and it doesn't have to return anything; if it's None then it returns a decorator function that will take the object obj, do its registration thing, and return it unchanged.

Now, that's a pretty complicated signature. It can still be done - at least defining the return value, not attaching it to the nullness of the second argument. But... is it worth? If this function is so complicated to describe, maybe the request of asking what type it is just makes this complexity to stand in its ugliness. Instead of insisting on the original idea and fighting Mypy to craft an overly complicated return value, splitting it into two simpler functions seemed to me the right choice. Now register()'s second argument is no more optional and its return value is always None; the decorator is just a separate method.

# The function is called one way
Adapter.register(int, adapt_number)

# The decorator another way
@Adapter.text(int)  # can be 'text()' or 'binary()'
def adapt_number(s):

It also makes sense from the naming point of view: register() is an active verb, text is a sort of declaration. The function can expose other optional arguments, for instance a scope where to register the adapter, whereas the decorator couldn't make use of them. They are two functions instead of one, but each one does only one thing, and with fewer parameters.

I hate when a program tells me my design is wrong. But maybe it has a point. Complicating the design for syntactic sugar maybe is not a healthy thing to do (it causes cancer of the semicolon someone said...)

ctypes integration

ctypes allows to bind to C functions defined in external libraries and it has its types annotation and automatic conversion to Python (or to segfault if you get it wrong).

Mypy doesn't know much about it, so even in a simple case such as:

# ctypes wrapper
PQhost = pq.PQhost
PQhost.argtypes = [PGconn_ptr]
PQhost.restype = c_char_p

# later on, a wrapper object
def host(self) -> bytes:
    return PQhost(self.pgconn_ptr)

it will complain of

Returning Any from function declared to return "bytes"

Unfortunately PQhost is a callable object, not a function, and it's of the same type of all the other functions with different signatures, so it cannot be annotated.

What helped in this situation was to define a stub, which is a .pyi file containing only the types and no definition, and it is the mechanism to add annotations to stuff that is not pure Python code; in this case:

def PQhost(arg1: Optional[PGconn_struct]) -> bytes: ...

Writing a stub to pretend that those callables are functions is simple enough. What is painful is having to define all the signatures pretty much twice: error prone and numbingly boring. This calls for the mythical lazy programmer which, given a boring task, writes a program to do it for them. It's simple enough to write a script inspecting the module containing the ctypes definition and generating the repetitive part of the stub, leaving to write manually only the delicate cases (e.g. to disambiguate between a pointer and an array): if you find it useful you can find the interface generator in this function, and here is the stub generated.

It found two bugs!

Most of my time was spent adding types to every function. A bit of refactoring here and there to remove variables reuse etc. A lot was spent learning how to use the whole thing. But apart from complaining about the lack of signatures, two errors reported by Mypy were actual bugs.

One was with the choice of the codec to use. There is a map of encodings from Postgres names to Python names which we use to convert the connection encoding to a Python codec. Some of the Postgres encodings are unknown to Python, and SQL_ASCII is a bit special (it actually means "no encoding") and it needs to be special-cased when reading from the db, so I left it as None in the encodings map.

But because of that None my code was wrong:

# for unknown encodings and SQL_ASCII be strict and use ascii
pyenc = pq.py_codecs.get(pgenc, "ascii")
self._codec = codecs.lookup(pyenc)

This bit of code wanted to deal both with a missing encoding and with SQL_ASCII, but it does it wrong. If pgenc is None then pyenc is None, not "ascii", and codecs.lookup() will go boom. What the above really needs to be is more:

 # for unknown encodings and SQL_ASCII be strict and use ascii
- pyenc = pq.py_codecs.get(pgenc, "ascii")
+ pyenc = pq.py_codecs.get(pgenc) or "ascii"
 self._codec = codecs.lookup(pyenc)

Another bug was a pasto: every connection and every cursor have two mappings: adapters and casters to customize the conversions respectively to and from the database, and there are global mappings as a fallback. Sure enough I copypasted some of it, and sure enough I forgot to change a bit:

- where = context.adapters if context is not None else Typecaster.globals
+ where = context.casters if context is not None else Typecaster.globals
  where[oid, format] = caster
  return caster

But because I had added something similar to the following definitions:

class Typecaster:
    globals: Dict[Tuple[Oid, Format], TypecasterFunc] = {}

class Connection:   # a context
    def __init__(self):
        adapters: Dict[Tuple[type, Format], AdapterFunc] = {}
        casters: Dict[Tuple[Oid, Format], TypecasterFunc] = {}

the type system couldn't find a type for where to reconcile the two sides of the if: even if both values are dicts they won't contain the same types.

Now, I can find plenty of excuses to justify these two errors: they were in a part of the codebase still stubbed out because it's a design phase and tests for them are not written yet (at the moment there are only two adapters, for string and numbers, just to play with the whole adaptation stack). I'm sure tests finding the issues would have been added later during development. But my excuses don't change an important matter: those bugs were there. In TDD, if you don't have a test, you don't have anything: your program is wrong pretty much by definition.


What I have experienced is that most people say "tests are useful", usually followed by "but we don't have time to write them". Or something funny like "let's write this quickly and release, then we'll write the tests".

If this is the case Mypy can actually be quite useful: adding types is a quick matter once you have familiarity with the type system and if you annotate as the code as you write it instead of, as I did, as a second pass. It's easy to run Mypy in CI and make sure the program still "compiles ok". You can make sure that a whole program is completely type-checked much more easily than to make sure there are enough tests (no, code-coverage is no such a measure).

Is Mypy enough to be confident about a program's behaviour? Not at all, not in my book, as much as bugs do exist in compiled programs - boy if exist. Mypy+pytest is a great team though: they pretty much complement each other, one giving a shred of a sort of formal proof that what you wrote is consistent, the other helping to deal with testing things for real.

Is the extra time spent writing annotation and wrestling the type system a good investment? I think in itself you can dismiss the use of Mypy only if your test-writing practice is between "high" and "obsessive". But if you also factor in other side effects such as having to think harder about types and functions interfaces, having the signatures as part of the documentation ("show me your tables..."), then the benefits start stacking up and I think it makes the use of Mypy quite compelling.

After my annotation marathon I have been thinking for a while if it was worth: I did it as an experiment, giving myself the option to abandon that branch, because I've only started with this project and I don't want annotating to become a time sink as the codebase grow. But I think writing down this write-up helped me clarifying my ideas about Mypy, and I think it will stay here and be a --strict part of psycopg3.

But now, please, just go and write those tests.