Tupasto

Thoughts and opinions on Software Development and Music Production.

Python Tuple Syntax Is Confusing

23 Dec, 2019

Creating a list or a set with only one element, in Python, is pretty straight forward:

# one element list.
["this is a list"]

# one element set.
{"this is a set"}

However, that’s not the case if you want to create a tuple with only one element:

("sorry! this is not a tuple")

Yup, unfortunately the way you create a one element tuple does not conform with the rest of the data structures in Python which you can create using “special” characters.

If you want to create a tuple with only one element you must not forget the final comma:

("this is now a tuple thanks to the comma",)

I think this is bad. First, because it’s different from all the other listed above, and second, because a string, as well as a list, a set or a tuple, is also enumerable. So, for functions that are expecting enumerables, if you provide them with a string they will “sort of” work.

A coworker of mine justified the behaviour with:

“But you need the parenthesis in order to separate math operations. How would you differentiate between a tuple with one int and a parenthesis which is only wrapping a mathematical operation?”

Namely:

(2)

(1 + 1)

Which is an understable argument seeing as we still need the ability to use paranthesis in mathematical operations but, the truth is, when I think of the first I think we’re creating a tuple, however, its result is the same as the second expression.

Getting bit by it!

Why did I bother making a blog post about this? Mostly because I got bitten by this little Python behaviour at work.

I was updating one codebase’s tests, which uses aioresponse in order to mock web requests when working with aiohttp. We were using it as a context manager, and you can provide a passthrough parameter to it, this should be an iterable, which contains all the hosts that aioresponse must not mock. Since we were doing service level tests we don’t want to mock requests to the actual service’s URL.

This is how the specific code that uses aioresponse was when I started working on it:

with aioresponses(passthrough=(service_host, external_url)) as responses:
    yield responses

service_host refers to the URL of the service being updated/maintained, while external_url was the URL of another service in which the one being updated depends on.

I decided to update this so as to mock the calls to the external_url since we want to avoid, as much as possible, having to make HTTP requests to other services when doing service level tests. As such, I updated the passthrough parameter as seen in the code below:

with aioresponses(passthrough=(service_host)) as responses:
    yield responses

As you can probably notice, I forgot to add the final comma…

So, now instead of providing a tuple to passthrough we were providing a string. However, since a string can also be iterated the code wasn’t broken per say. When running the test suite no errors were thrown but the test suite didn’t run successfully.

When looking at aioresponse's codebase we noticed that it iterates through the passthrough value and checks if the current URL starts with any of the values in passthrough.

Since we only passed a string, when iterating through passthrough the first value will be "h". Can you see where this is going?

I’ll leave a snippet here to give you and idea of what I’m talking about:

passthrough = ("http://do_not_mock.com")
current_url = "http://mock_me.com"

for url in passthrough:
  if current_url.startswith(url):
    # Do not mock current_url!

Basically no HTTP requests were being mocked because all the URLs used in the test suite start with "http" (well, that’s a first!) and "http...".startswith("h") does return True. All of this, and three hours lost, because of a sneaky comma!!!

Conclusion

In conclusion, I know there’s probably not much of an use for a one element tuple but it’s a bit frustating that it relies on that single command at the end, which a lot of people, like me, might accidentaly delete while messing around a codebase.

I also know that Python is not the only language to have this kind of “issues” (not really an issue but you get what I’m saying) so I’m not trying to bash on Python or stating that aioresponse should be updated to handle this scenario,

I just think we could have better tuple syntax in Python so as to avoid this kind of confusion.