Is there a one-line way of deleting a key from a dictionary without raising a KeyError
?
if 'key' in my_dict:
del my_dict['key']
To delete a key regardless of whether it is in the dictionary, use the two-argument form of dict.pop()
:
my_dict.pop('key', None)
This will return my_dict[key]
if key
exists in the dictionary, and None
otherwise. If the second parameter is not specified (i.e. my_dict.pop('key')
) and key
does not exist, a KeyError
is raised.
To delete a key that is guaranteed to exist, you can also use:
del my_dict['key']
This will raise a KeyError
if the key is not in the dictionary.
Specifically to answer "is there a one line way of doing this?"
if 'key' in my_dict: del my_dict['key']
...well, you asked ;-)
You should consider, though, that this way of deleting an object from a dict
is not atomic—it is possible that 'key'
may be in my_dict
during the if
statement, but may be deleted before del
is executed, in which case del
will fail with a KeyError
. Given this, it would be safest to either use dict.pop
or something along the lines of
try:
del my_dict['key']
except KeyError:
pass
which, of course, is definitely not a one-liner.
pop
is a definitely more concise, though there is one key advantage of doing it this way: it's immediately clear what it's doing.
try/except
statement is more expensive. Raising an exception is slow.
try
is marginally faster, though if it doesn't, try
is indeed a good deal slower. pop
is fairly consistent but slower than all but try
with a non-present key. See gist.github.com/zigg/6280653. Ultimately, it depends on how often you expect the key to actually be in the dictionary, and whether or not you need atomicity—and, of course, whether or not you're engaging in premature optimization ;)
if 'key' in mydict: #then del...
. I needed to pull out a key/val from a dict to parse correctly, pop was not a perfect solution.
It took me some time to figure out what exactly my_dict.pop("key", None)
is doing. So I'll add this as an answer to save others googling time:
pop(key[, default]) If key is in the dictionary, remove it and return its value, else return default. If default is not given and key is not in the dictionary, a KeyError is raised.
dict.pop?
in IPython.
del my_dict[key]
is slightly faster than my_dict.pop(key)
for removing a key from a dictionary when the key exists
>>> import timeit
>>> setup = "d = {i: i for i in range(100000)}"
>>> timeit.timeit("del d[3]", setup=setup, number=1)
1.79e-06
>>> timeit.timeit("d.pop(3)", setup=setup, number=1)
2.09e-06
>>> timeit.timeit("d2 = {key: val for key, val in d.items() if key != 3}", setup=setup, number=1)
0.00786
But when the key doesn't exist if key in my_dict: del my_dict[key]
is slightly faster than my_dict.pop(key, None)
. Both are at least three times faster than del
in a try
/except
statement:
>>> timeit.timeit("if 'missing key' in d: del d['missing key']", setup=setup)
0.0229
>>> timeit.timeit("d.pop('missing key', None)", setup=setup)
0.0426
>>> try_except = """
... try:
... del d['missing key']
... except KeyError:
... pass
... """
>>> timeit.timeit(try_except, setup=setup)
0.133
del
and pop
from the first set of timings above)
If you need to remove a lot of keys from a dictionary in one line of code, I think using map() is quite succinct and Pythonic readable:
myDict = {'a':1,'b':2,'c':3,'d':4}
map(myDict.pop, ['a','c']) # The list of keys to remove
>>> myDict
{'b': 2, 'd': 4}
And if you need to catch errors where you pop a value that isn't in the dictionary, use lambda inside map() like this:
map(lambda x: myDict.pop(x,None), ['a', 'c', 'e'])
[1, 3, None] # pop returns
>>> myDict
{'b': 2, 'd': 4}
or in python3
, you must use a list comprehension instead:
[myDict.pop(x, None) for x in ['a', 'c', 'e']]
It works. And 'e' did not cause an error, even though myDict did not have an 'e' key.
map
and friends are now lazy and return iterators. Using map
for side-effects is generally considered poor practice; a standard for ... in
loop would be better. See Views And Iterators Instead Of Lists for more information.
[myDict.pop(i, None) for i in ['a', 'c']]
, as they offer a general alternative to map
(and filter
).
for ... in
loop.
map()
, which is often used for its side-effects. The recommended alternative in Python is the list comprehension, which in my opinion is still quite readable and cognitively light as a one-liner (see question). Used only for their side-effects, both constructs indeed result in a useless list, which can be inefficient. As of Python3, I'm not aware of a built-in function that can safely and elegantly iterate through a generator expression, without a costly byproduct e.g. loop(d.pop(k) for k in ['a', 'b'])
.
for k in ['a', 'b']: d.pop(k)
. Why do you need a different way to do it? If you need it to take up only one line that badly then you can put the second part of the for loop on the same line.
You can use a dictionary comprehension to create a new dictionary with that key removed:
>>> my_dict = {k: v for k, v in my_dict.items() if k != 'key'}
You can delete by conditions. No error if key
doesn't exist.
del dict[key]
or dict.pop(key)
will be faster in all cases.
del
and pop
are faster, but sometimes you just don't want to modify the original dictionary. It would make more sense if in the example above, the comprehension result was assigned to another variable.
Using the "del" keyword:
del dict[key]
We can delete a key from a Python dictionary by the some of the following approaches.
Using the del
keyword; it's almost the same approach like you did though -
myDict = {'one': 100, 'two': 200, 'three': 300 }
print(myDict) # {'one': 100, 'two': 200, 'three': 300}
if myDict.get('one') : del myDict['one']
print(myDict) # {'two': 200, 'three': 300}
Or
We can do like the following:
But one should keep in mind that, in this process actually it won't delete any key from the dictionary rather than making a specific key excluded from that dictionary. In addition, I observed that it returned a dictionary which was not ordered the same as myDict
.
myDict = {'one': 100, 'two': 200, 'three': 300, 'four': 400, 'five': 500}
{key:value for key, value in myDict.items() if key != 'one'}
If we run it in the shell, it'll execute something like {'five': 500, 'four': 400, 'three': 300, 'two': 200}
- notice that it's not the same ordered as myDict
. Again if we try to print myDict
, then we can see all keys including which we excluded from the dictionary by this approach. However, we can make a new dictionary by assigning the following statement into a variable:
var = {key:value for key, value in myDict.items() if key != 'one'}
Now if we try to print it, then it'll follow the parent order:
print(var) # {'two': 200, 'three': 300, 'four': 400, 'five': 500}
Or
Using the pop()
method.
myDict = {'one': 100, 'two': 200, 'three': 300}
print(myDict)
if myDict.get('one') : myDict.pop('one')
print(myDict) # {'two': 200, 'three': 300}
The difference between del
and pop
is that, using pop()
method, we can actually store the key's value if needed, like the following:
myDict = {'one': 100, 'two': 200, 'three': 300}
if myDict.get('one') : var = myDict.pop('one')
print(myDict) # {'two': 200, 'three': 300}
print(var) # 100
Fork this gist for future reference, if you find this useful.
if myDict.get('one')
to check if a key is present! It fails if myDict['one'] has a falsey value. Also, dicts don't have an inherent order, so it doesn't make sense to mention it.
You can use exception handling if you want to be very verbose:
try:
del dict[key]
except KeyError: pass
This is slower, however, than the pop()
method, if the key doesn't exist.
my_dict.pop('key', None)
It won't matter for a few keys, but if you're doing this repeatedly, then the latter method is a better bet.
The fastest approach is this:
if 'key' in dict:
del myDict['key']
But this method is dangerous because if 'key'
is removed in between the two lines, a KeyError
will be raised.
Dictionary data type has a method called dict_name.pop(item)
and this can be used to delete a key:value pair from a dictionary.
a={9:4,2:3,4:2,1:3}
a.pop(9)
print(a)
This will give the output as:
{2: 3, 4: 2, 1: 3}
This way you can delete an item from a dictionary in one line.
Another way is by using items() + dict comprehension.
items() coupled with dict comprehension can also help us achieve the task of key-value pair deletion, but it has the drawback of not being an in place dict technique. Actually a new dict if created except for the key we don’t wish to include.
test_dict = {"sai" : 22, "kiran" : 21, "vinod" : 21, "sangam" : 21}
# Printing dictionary before removal
print ("dictionary before performing remove is : " + str(test_dict))
# Using items() + dict comprehension to remove a dict. pair
# removes vinod
new_dict = {key:val for key, val in test_dict.items() if key != 'vinod'}
# Printing dictionary after removal
print ("dictionary after remove is : " + str(new_dict))
Output:
dictionary before performing remove is : {'sai': 22, 'kiran': 21, 'vinod': 21, 'sangam': 21}
dictionary after remove is : {'sai': 22, 'kiran': 21, 'sangam': 21}
just create a copy of your dictionary .
newMy_dict = my_dict.copy()
if 'key' in newMy_dict :
del newMy_dict['key']
This way , you can control exception.
I prefer the immutable version
foo = {
1:1,
2:2,
3:3
}
removeKeys = [1,2]
def woKeys(dct, keyIter):
return {
k:v
for k,v in dct.items() if k not in keyIter
}
>>> print(woKeys(foo, removeKeys))
{3: 3}
>>> print(foo)
{1: 1, 2: 2, 3: 3}
Single filter on key
return "key" and remove it from my_dict if "key" exists in my_dict
return None if "key" doesn't exist in my_dict
this will change my_dict in place (mutable)
my_dict.pop('key', None)
Multiple filters on keys
generate a new dict (immutable)
dic1 = {
"x":1,
"y": 2,
"z": 3
}
def func1(item):
return item[0]!= "x" and item[0] != "y"
print(
dict(
filter(
lambda item: item[0] != "x" and item[0] != "y",
dic1.items()
)
)
)
You could also use filter
with lambda
:
>>> d = {'a': 1, 'b': 2, 'c': 3}
>>> dict(filter(lambda x: x[0] != 'a', d.items()))
{'b': 2, 'c': 3}
>>>
my_dict.pop('key', None)
is the best answer
Try this
if key in data:
del data[key]
Success story sharing
pop()
overdel
: it returns the value for that key. This way you can get and delete an entry from a dict in one line of code.try
/except
solution if the key does not exist. You might find one or the other easier to read, which is fine. Both are idiomatic Python, so choose whatever you prefer. But claiming that this answer is more complex or inefficient simply makes no sense.self
, so it would be rather surprising if this one did.