Table of contents
- Design recommendations
- Keep I/O separate
- Duck typing is a good idea
- Consider: can this just be a function?
- Avoid changing state
- Consider: do I really want a custom class?
- Static typing is verbose, but makes code more readable
- Permissiveness isn’t always convenient
- Write useful error messages
- Write for readability
- Complexity is always conserved
One of the biggest impediments to reuse of scientific code is when I/O code—assuming certain file locations, names, formats, or layouts—is interspersed with scientific logic.
I/O-related functions should only perform I/O. For example, they should take in a filepath and return a numpy array, or a dictionary of arrays and metadata. The valuable scientific logic should be encoded in functions that take in standard data types and return standard data types. This makes them easier to test, maintain when data formats change, or reuse for unforeseen applications.
Duck typing treats objects based on what they can do, not based on what type they are. “If it walks like a duck and it quacks like a duck, then it must be a duck.”
Python in general and scientific Python in particular leverage interfaces (also known as Protocols) to support interoperability and reuse. For example, it is possible to pass a pandas DataFrame to the
numpy.sum function even though pandas was created long after
numpy.sum. This is because
numpy.sum avoids assuming it will be passed specific data types; it accepts any object that provides the right methods (interfaces). Where possible, avoid
isinstance checks in your code, and try to make your functions work on the broadest possible range of input types.
Not everything needs to be object-oriented. Object-oriented design needs to follow the same principles as other code, like modularity, and be well tested. If you can get away with writing functions processing existing datatypes like a DataFrame, do so.
It is better to have 100 functions operate on one data structure than 10 functions on 10 data structures.
– From ACM’s SIGPLAN publication, (September, 1982), Article “Epigrams in Programming”, by Alan J. Perlis of Yale University.
A popular talk, “Stop Writing Classes”, illustrates how some situations that seem to lend themselves to object-oriented programming are much more simply handled using functions. The biggest danger of reaching for OO design when it’s not needed is the following: changing states.
It is often tempting to invent a custom class to express a workflow, along these lines.
data = Data()
That’s easy and simple. Unless you forget a step. Oh, yeah, and static analysis tools can’t tell you if you forget a step, the API doesn’t statically “know” that
.prepare() is required, for example. Tab completion tells you that
.plot() is valid immediately. The underlying problem is that
Data has a implicit changing state, and not all operations are valid in all possible states.
One alternative replace
Data with multiple immutable classes representing the state at each step.
empty_data = EmptyData()
loaded_data = empty_data.load_data()
prepared_data = loaded_data.prepare()
computed_data = prepared_data.do_calculations()
We could avoid naming the temporaries, too:
computed_data = EmptyData().load_data().prepare().do_calculations()
These classes don’t have to be immutable. Maybe you can load more data to loaded data. But they are easier to use correctly when they at least avoid a mutating state that makes subsets of available operations (i.e. methods) invalid. Note that tab completion in this case would show exactly the allowed set of operations each time.
Using built-in Python types (
str) and standard scientific Python types like NumPy array and Pandas DataFrame makes code interoperable. It enables data to flow between different libraries smoothly, and to be extended in unforeseen ways.
As an example, the widely-used library scikit-image initially experimented with using an
Image class, but ultimately decided that it was better to use plain old NumPy arrays. All scientific Python libraries understand NumPy arrays, but they don’t understand custom classes, so it is better to pass application-specific metadata alongside a standard array than to try to encapsulate all of that information in a new, bespoke object. (Modern NumPy uses Protocols to make this type of use much easier).
When you want to group data together into one object for convenience, consider dataclasses.
from dataclasses import dataclass
Code with static types has a lot of extra characters, but it provides more information to the reader;
timestamp: int or
timestamp: float provides valuable information to the reader about the types that might be hard to infer from the variable name alone, and therefore what is valid and what isn’t. It also can be verified in static typing checkers, like MyPy, so that it is more likely to be correct than types in docstrings.
There’s another benefit: if you design your code with types in mind, you’ll tend toward simpler, less dynamic designs with a clearly defined expected usage. You’ll remember (well, at least you are more likely to remember) to handle special cases like lists vs strings or values that could also be
When using static typing, duck typing is expressed via Protocols. These should be strongly preferred over older solutions like inheritance or ABCs if possible, as they trade a little extra code to remove dependencies between objects.
Overly permissive code can lead to very confusing bugs. If you need a flexible user-facing interface that tries to “do the right thing” by guessing what the users wants, separate it into two layers: a thin “friendly” layer on top of a “cranky” layer that takes in only exactly what it needs and does the actual work. The cranky layer should be easy to test; it should be constrained about what it accepts and what it returns. This layered design makes it possible to write many friendly layers with different opinions and different defaults.
When it doubt, make function arguments required. Optional arguments are harder to discover and can hide important choices that the user should know that they are making.
Exceptions should just be raised: don’t catch them and print. Exceptions are a tool for being clear about what the code needs and letting the caller decide what to do about it. Application code (e.g. GUIs) should catch and handle errors to avoid crashing, but library code should generally raise errors unless it is sure how the user or the caller wants to handle them.
Be specific. Include what the wrong value was, what was wrong with it, and perhaps how it might be fixed. For example, if the code fails to locate a file it needs, it should say what it was looking for and where it looked.
Unless you are writing a script that you plan to delete tomorrow or next week, your code will probably be read many more times than it is written. And today’s “temporary solution” often becomes tomorrow’s critical code. Therefore, optimize for clarity over brevity, using descriptive and consistent names.
Complexity is always conserved and is strictly greater than the system the code is modeling. Attempts to hide complexity from the user frequently backfire.
For example, it is often tempting to hide certain reused keywords in a function, shortening this:
normalize: bool = True,
beginning: int = 0,
end: int | None = None,
) -> np.ndarray: ...
def get_image(filename: Path, **kwargs: Any) -> np.ndarray: ...
Although the interface appears to have been simplified through hidden keyword arguments, now the user needs to remember what the
kwargs are or dig through documentation to better understand how to use them. You also lose static typing.
Because new science occurs when old ideas are reapplied or extended in unforeseen ways, scientific code should not bury its complexity or overly optimize for a specific use case. It should expose what complexity there is straightforwardly.
Even better, you should consider using “keyword-only” arguments, introduced in Python 3, which require the user to pass an argument by keyword rather than position.
normalize: bool = True,
beginning: int = 0,
end: int | None = None
) -> np.ndarray: ...
Every argument after the
* is keyword-only. Therefore, the usage
get_image('thing.png', False) will not be allowed; the caller must explicitly type
get_image('thing.png', normalize=False). The latter is easier to read, and it enables the author to insert additional parameters without breaking backward compatibility.
Similarly, it can be tempting to write one function that performs multiple steps and has many options instead of multiple functions that do a single step and have few options. The advantages of “many small functions” reveal themselves in time:
- Small functions are easier to explain and document because their behavior is well-scoped.
- Small functions can be tested individually, and it is easy to see which paths have and have not yet been tested.
- It is easier to compose a function with other functions and reuse it in an unanticipated way if its behavior is well-defined and tightly scoped. This is the UNIX philosophy, “Do one thing and do it well.”
- The number of possible interactions between arguments goes up with the number of arguments, which makes the function difficult to reason about and test. In particular, arguments whose meaning depends on other arguments should be avoided.
Functions should return the same kind of thing no matter what their arguments, particularly their optional arguments. Violating “return type stability” puts a burden on the function’s caller, which now must understand the internal details of the function to know what type to expect for any given input. That makes the function harder to document, test, and use. Python does not enforce return type stability, but we should try for it anyway. If you have a function that returns different types of things depending on its inputs, that is a sign that it should be refactored into multiple functions.
Python is incredibly flexible. It accommodates many possible design choices. By exercising some restraint and consistency with the scientific Python ecosystem, Python can be used to build scientific tools that last and grow well over time.