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" and "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 enumerate() instead:
[i for i,x in enumerate([1,2,3,2]) if x==2] # => [1, 3]
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" and "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 enumerate() instead:
[i for i,x in enumerate([1,2,3,2]) if x==2] # => [1, 3]
If you want to find one element or None use default in next, it won't raise StopIteration if the item was not found in the list:
first_or_default = next((x for x in lst if ...), None)
Videos
Doesn't it make sense for lists to have a find method?
class MyList(list):
def find(self, func):
for i in self:
if func(i):
return i
return None
# or raise ValueError?
users = [
{
'id': 1,
'name': 'john',
},
{
'id': 2,
'name': 'anna',
},
{
'id': 3,
'name': 'bruce',
}
]
my_list = MyList(users)
user_2 = my_list.find(lambda i: i['id'] == 2)
print(user_2) # {'id': 2, 'name': 'anna'}I don't know why or maybe is buried in some PEP somewhere, but i do know 2 very basic "find" method for lists, and they are array.index() and the in operator. You can always make use of these 2 to find your items. (Also, re module, etc)
I think the rationale for not having separate 'find' and 'index' methods is they're not different enough. Both would return the same thing in the case the sought item exists in the list (this is true of the two string methods); they differ in case the sought item is not in the list/string; however you can trivially build either one of find/index from the other. If you're coming from other languages, it may seem bad manners to raise and catch exceptions for a non-error condition that you could easily test for, but in Python, it's often considered more pythonic to shoot first and ask questions later, er, to use exception handling instead of tests like this (example: Better to 'try' something and catch the exception or test if its possible first to avoid an exception?).
I don't think it's a good idea to build 'find' out of 'index' and 'in', like
if foo in my_list:
foo_index = my_list.index(foo)
else:
foo_index = -1 # or do whatever else you want
because both in and index will require an O(n) pass over the list.
Better to build 'find' out of 'index' and try/catch, like:
try:
foo_index = my_list.index(foo)
catch ValueError:
foo_index = -1 # or do whatever else you want
Now, as to why list was built this way (with only index), and string was built the other way (with separate index and find)... I can't say.