Python Check dictionary object for correct keys throw custom exception

How about writing all the keys you want to check for in a tuple?

def mymethod(temp_dict):
    mykeys = ('key1', 'key2', 'key3', ..., 'keyN')
    for k in mykeys:
        if k not in temp_dict:
            raise CustomException("{} missing from dictionary".format(k))

In this way anytime you want to add a new key to check for you can just add it to the tuple instead of duplicating code.

One possibility would be to use a schema library such as voluptuous, in which you define your dictionary schema, by defining which keys are required and which are optional and even which acceptable types the keys can have.

On the other hand I have this nifty little function that I've used so far if you would only need to validate if all required keys are present. First we define a list containing all the required keys. Then we check if the given dictionary has all these required keys present using the all method.

def validate_params(params, req_keys=[]):                                           
    """                                                                      
    Verify if all required keys are present in the dictionary passed   
    to this method                                                           

    :param params: Parameter Dictionary to check
    :param req_keys: These Keys are required in the dictionary
    :rtype: Boolean                                                          
    :returns: True or False                                                  
    """                                                                      
    if all(_ in params for _ in req_keys):                       
        return True                                                          
    raise CustomException("Not all keys present in dictionary")                                                             

I've added a fiddle on pythonfiddle.com.

The most Pythonic way to do this is with a loop:

>>> class CustomException(Exception):
...     pass
...
>>> keys = 'key0 key1 key2 key3'.split()
>>> my_dict = {k: v for k, v in zip(keys, xrange(4))}
>>>
>>> for k in keys:
...     if not k in my_dict:
...         raise CustomException('Key %s not in dictionary.' % k)
...
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
__main__.CustomException: Key key4 not in dictionary.
>>>

However, an even more Pythonic way to do this would be for your function to simply do a try/except for each required key in your function as you use it. For example:

>>> keys = 'key0 key1 key2 key3 key4'.split()
>>>
>>> def f(my_dict):
...     for key in keys:
...         try:
...             print k, my_dict[k]
...         except KeyError:
...             raise CustomException('Key %s not in dictionary.' % k)
...
>>> my_dict = {k: v for k, v in zip(keys[:-1], xrange(len(keys) - 1))}
>>> assert my_dict == {'key0': 0, 'key1': 1, 'key2': 2, 'key3': 3}  # Note no 'key4'
>>> f(my_dict)
key4
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in f
__main__.CustomException: Key key4 not in dictionary.
>>>

This is known as the "Easier to Ask Forgiveness than Permission" (or: "EAFP") principle. Proceed with what you plan to do until something stops you.

Use any function:

if any(k not in temp_dict for k in ('key', 'key1', 'key2', 'key3')):
    raise CustomException("Some keys are missing from dict")