Check if a given key already exists in a dictionary

Check if a given key already exists in a dictionary

To get the idea how to do that we first inspect what methods we can call on dictionary.

Here are the methods:

d={'clear':0, 'copy':1, 'fromkeys':2, 'get':3, 'items':4, 'keys':5, 'pop':6, 'popitem':7, 'setdefault':8, 'update':9, 'values':10}

Python Dictionary clear()        Removes all Items
Python Dictionary copy()         Returns Shallow Copy of a Dictionary
Python Dictionary fromkeys()     Creates dictionary from given sequence
Python Dictionary get()          Returns Value of The Key
Python Dictionary items()        Returns view of dictionary (key, value) pair
Python Dictionary keys()         Returns View Object of All Keys
Python Dictionary pop()          Removes and returns element having given key
Python Dictionary popitem()      Returns & Removes Element From Dictionary
Python Dictionary setdefault()   Inserts Key With a Value if Key is not Present
Python Dictionary update()       Updates the Dictionary
Python Dictionary values()       Returns view of all values in dictionary

The brutal method to check if the key already exists may be the get() method:

d.get("key")

The other two interesting methods items() and keys() sounds like too much of work. So let's examine if get() is the right method for us. We have our dict d:

d= {'clear':0, 'copy':1, 'fromkeys':2, 'get':3, 'items':4, 'keys':5, 'pop':6, 'popitem':7, 'setdefault':8, 'update':9, 'values':10}

Printing shows the key we don't have will return None:

print(d.get('key')) #None
print(d.get('clear')) #0
print(d.get('copy')) #1

We use that to get the information if the key is present or no. But consider this if we create a dict with a single key:None:

d= {'key':None}
print(d.get('key')) #None
print(d.get('key2')) #None

Leading that get() method is not reliable in case some values may be None.

This story should have a happier ending. If we use the in comparator:

print('key' in d) #True
print('key2' in d) #False

We get the correct results.

We may examine the Python byte code:

import dis
dis.dis("'key' in d")
#   1           0 LOAD_CONST               0 ('key')
#               2 LOAD_NAME                0 (d)
#               4 COMPARE_OP               6 (in)
#               6 RETURN_VALUE

dis.dis("d.get('key2')")
#   1           0 LOAD_NAME                0 (d)
#               2 LOAD_METHOD              1 (get)
#               4 LOAD_CONST               0 ('key2')
#               6 CALL_METHOD              1
#               8 RETURN_VALUE

This shows that in compare operator is not just more reliable, but even faster than get().

Another way of checking if a key exists using Boolean operators:

d = {'a': 1, 'b':2}
keys = 'abcd'

for k in keys:
    x = (k in d and 'blah') or 'boo'
    print(x)

This returns

>>> blah
>>> blah
>>> boo
>>> boo

Explanation

First, you should know that in Python, 0, None, or objects with zero length evaluate to False. Everything else evaluates to True. Boolean operations are evaluated left to right and return the operand not True or False.

Let's see an example:

>>> 'Some string' or 1/0
'Some string'
>>>

Since 'Some string' evaluates to True, the rest of the or is not evaluated and there is no division by zero error raised.

But if we switch the order 1/0 is evaluated first and raises an exception:

>>> 1/0 or 'Some string'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>>

We can use this for pattern for checking if a key exists.

(k in d and 'blah')

does the same as

if k in d:
    'blah'
else:
    False

This already returns the correct result if the key exists, but we want it to print 'boo' when it doesn't. So, we take the result and or it with 'boo'

>>> False or 'boo'
'boo'
>>> 'blah' or 'boo'
'blah'
>>>

You can use a for loop to iterate over the dictionary and get the name of key you want to find in the dictionary. After that, check if it exist or not using if condition:

dic = {'first' : 12, 'second' : 123}
for each in dic:
    if each == 'second':
        print('the key exists and the corresponding value can be updated in the dictionary')