Python pitfalls for functional programmers

What I learned from doing the Advent of Code 2021 in Python with a Scala background

I have been using Python on and off for scripting, glue programs and simple ML notebooks. To get a deeper understanding of the language and evaluate how it would scale on larger programs, I decided to use it for the Advent of code 2021.

This post contains the result of my observations: as I have been using mostly functional languages recently, I had quite a few surprises with Python. Although it has functional constructs, some of its non functional behaviours initially through me off and I thought it was worth sharing.

As a development environment I used VS code with the Python extension. VS code works very well on both Linux and Mac, it is lightweight and includes a step by step debugger.

Side effects and mutable collections !

Coming with functional habits, I took it for granted that collections were immutable and that operations on collections returned a ‘new’ copy of the collection. This is obviously a mental model, the implementation in functional languages is optimised. Based on the fact that collections are immutable, there is possible memory reuse.

In Python, the only method on a list that returns a list is copy(), c.f. The main drawback of that approach is that it is not possible to ‘chain’ operations.

For example, if I want the first 5 smallest integers of a list, in Scala I would chain sort() and take() as in l.sort().take(5). In Python I would to first ‘sort()’ (and mutate) the list, then take the first elements.


Immutable collections and whether they contain a value or a reference to value can also introduce subtle bugs.

Here is an interesting example:

>>> m = [[False] * 3] * 3
>>> m
[[False, False, False], [False, False, False], [False, False, False]]
>>> m[0][0]=True
>>> m
[[True, False, False], [True, False, False], [True, False, False]]

When m is created, it looks like a 2 dimensional boolean array (well list of lists to be precise !). But when we try to modify one element (m[0][0]), we realise there is actually only 1 row which is referenced 3 times in the top level list !

This is because the operator * behaves differently whether the list on the left contains:

  • either values, like with [False] * 3 where a copy of the value False is created
  • or references, like with [[False False False] * 3] where two new references to [False False False] are used, not a copy of the list.

This behaviour is consistent with the semantic of the assignment operator (=) but it was surprising none the less.

Actually Python does have some sort of immutable collections in the form of ‘tuples’. They cannot easily be used as immutable lists because there is only one operation possible on them.

With the ‘+’ operator it is possible to create a new tuple from an existing one and an element to the list.

For all other operations which would return a new tuple (like sort()), they need to be converted to a list and then a new tuple needs to be constructed.

Generators !

Python generators are a generalization of iterators) and as such they are inherently not functional: the next() operation on an iterator can return a different value every time it is called.

They are used extensively in Python as they perform well, even in ‘functional’ modules like functools.

For example, both map() and reduce() return an object which is an iterator. That means the resulting collection can only be ‘used’ (or more precisely iterated) only once.

It is very clear in the following session where the first sum(l2) returns the expected value (12=1*2+3*2+3*2) but the second call returns 0 because there are no more values to iterate, it is equivalent to evaluating sum([]).

$ python3
>>> l = [1,2,3]
>>> l2 = map(lambda e:e*2,l)
>>> sum(l2)
>>> sum(l2)
>>> type(l2)
<class 'map'>

l2 is not a list but an map class that implements an iterator.

Another side effect of this implementation (pardon the pun ;-)) is that evaluating some expressions while debugging will change the state of the program. For example, evaluating sum(l2) in the debugger will ‘run’ the iterator.

As a consequence, to make the code more readable (or at least behaves more like I was expecting), I used list comprehensions as they make explicit the result is an iterator. They can be used instead of map(), filter() and even flatmap() for some limited cases. That also makes the code more Python idiomatic.

The change is quite visible in my Advent of code 2021 solutions.

Dynamic typing !

Dynamic typing does exist in some functional languages (like Lisp or Scheme) but as the most ones like Scala or Haskell are statically typed, it is another difference that is worth keeping in mind.

One of the main drawbacks of dynamic typing is that quite a few errors are only caught when running the program (especially when refactoring). For example, if I forget the parenthesis in l.sort, the code will run but the list will not be sorted:

>>> l = [3,1,4]
>>> l.sort
<built-in method sort of list object at 0x7fe670a21cc0>
>>> l
[3, 1, 4]

As a consequence, I started adding more unit tests, especially to test functions/methods input parameters. It is not great when the project gets larger as more unit tests mean more code to keep up to date which clearly has a cost.

Another place where dynamic typing has an impact is that it blurs the line between tuples and lists. In statically typed functional languages, all elements of a list have the same type. The main advantage of a tuple is that it allows elements of different types.

I deliberately ignore inheritance that allow ‘Object’ list in Java as all the casting is very impractical and leads to buggy code. I will also not discuss HList as they are not (yet?) mainstream.

In Python, a list can contain elements of any type (but is mutable) like a tuple (which is immutable). It is important to keep in mind when choosing one other the other.


Python was easy to use (especially with VS Code and its integrated debugger), quick to learn and the libraries are very mature and efficient. A nice surprise was the implementation of integer, which has unlimited (well only by the memory !) precision.

I’m going to keep Python as a useful tool for quick prototyping/scripting but for most projects I still prefer to use a language with static typing (especially one using a Hindley-Milner type system) as the compiler catches a lot of issues and the code is still very readable.



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