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I use the following to check if
item is in
if item in my_list: print("Desired item is in list")
But sometimes, it doesn’t find the item. Why is this?
if item in my_list:" the most "pythonic" way of finding an item in a list?
As for your first question: "
if item is in my_list:" is perfectly fine and should work if
item equals one of the elements inside
my_list. The item must exactly match an item in the list. For instance,
"ABC" do not match. Floating point values in particular may suffer from inaccuracy. For instance,
1 - 1/3 != 2/3.
As for your second question: There’s actually several possible ways if "finding" things in lists.
Checking if something is inside
This is the use case you describe: Checking whether something is inside a list or not. As you know, you can use the
in operator for that:
3 in [1, 2, 3] # => True
Filtering a collection
That is, finding all elements in a sequence that meet a certain condition. You can use list comprehension or generator expressions for that:
matches = [x for x in lst if fulfills_some_condition(x)] matches = (x for x in lst if x > 6)
The latter will return a generator which you can imagine as a sort of lazy list that will only be built as soon as you iterate through it. By the way, the first one is exactly equivalent to
matches = filter(fulfills_some_condition, lst)
in Python 2. Here you can see higher-order functions at work. In Python 3,
filter doesn’t return a list, but a generator-like object.
Finding the first occurrence
If you only want the first thing that matches a condition (but you don’t know what it is yet), it’s fine to use a for loop (possibly using the
else clause as well, which is not really well-known). You can also use
next(x for x in lst if ...)
which will return the first match or raise a
StopIteration if none is found. Alternatively, you can use
next((x for x in lst if ...), [default value])
Finding the location of an item
For lists, there’s also the
index method that can sometimes be useful if you want to know where a certain element is in the list:
[1,2,3].index(2) # => 1 [1,2,3].index(4) # => ValueError
However, note that if you have duplicates,
.index always returns the lowest index:……
[1,2,3,2].index(2) # => 1
If there are duplicates and you want all the indexes then you can use
[i for i,x in enumerate([1,2,3,2]) if x==2] # => [1, 3]