Can't understand the line of code of Python

Question

I am a beginner and learning flask. While doing an exercise, I can't understand some code. Can anyone explain me the line with for loop?

@app.route("/details/<int:pet_id>")
def pet_details(pet_id):
    pet = next((pet for pet in pets if pet["id"] == pet_id), None) 
    if pet is None: 
        abort(404, description="No Pet was Found with the given ID")
    return render_template("details.html", pet = pet)

I can't understand this line from code

    pet = next((pet for pet in pets if pet["id"] == pet_id), None)

That line is a really complicated way of saying "get the pet with this id". — John Gordon, Sep 08 '20 at 17:02
@JacobIRR ... I dont think pets is a dictionary... its a list of dictionaries ... also what would you expect pets.get('id') to be ... i assume he is trying to get a specific pet with a specific id — Joran Beasley, Sep 08 '20 at 17:02
This might be informative: https://stackoverflow.com/questions/20496536/how-do-python-set-comprehensions-work/20497109#20497109 — Mate Mrše, Sep 08 '20 at 17:06
@JacobIRR. next requires an iterator object. If in your scenario `pets.get('id')` is not an iterator, next would break it. Much easier to do `pets.get('id', None)` — M Z, Sep 08 '20 at 17:14

score 3 · Accepted Answer · answered Sep 08 '20 at 17:00

next gets the next item from a generator, which in this case is (pet for pet in pets if pet["id"] == pet_id). This generator will reduce your list/collection of pet objects to only those whose id is the same as the requested pet_id.

The None here is defaulting: if the generator has no next element, it'll typically throw a StopIteration exception, but in this case, next will handle the exception and instead return a None object instead of throwing an error

Can't understand the line of code of Python

1 Answers1