#PYTHON(DAY12)dictionaries

 

#PYTHON(DAY12)

dictionaries

• dictionaries are ‘key’ & ‘value’ pairs enclosed in ‘{}’
• ‘key’ and ‘value’ are seperated by ‘:’ while ‘key and value’ pairs are seperated by ‘;’
• ‘keys’ are mapped to their ‘values’
• in dictionaries, ‘keys’ are unique, we can not have duplicate keys, we can have duplicate values
• dictionaries are unordered & mutable. ‘keys’ of dictionaries can be used to iterate over values
• ‘{}’ is an empty dictionary
• dict() is class of dictionary

creating a dictionary

x = {} # empty dictionary
print(x, type(x))

x = {'key1':1,
     'key2':2,
     'l1':'eleven',
      3:'three',
     'key1':100,
     'key3':200
    }
print(x,type(x))

And the output is:

{} <class 'dict'>
{'key1': 100, 'key2': 2, 'l1': 'eleven', 3: 'three', 'key3': 200} <class 'dict'>

Accessing values using keys

x['key1']

And the output is:
<function print>

x['key2']

And the output is:
[1, 2, 3]

x['key1'](type(['key2']))

And the output is:
<class 'list'>

dictionary methods

print(dir(x))

And the output is:

['__class__', '__class_getitem__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__ior__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__or__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__ror__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values']

update()

• update method is used to add ‘key’ and ‘value’ pair
• syntax — -> x.update({new_key:value})

x.update({'key6':100})
print(x)

And the output is:
{'key1': <built-in function print>, 'key2': [1, 2, 3], 'key3': (11, 12, 13), 'key4': {4, 5, 6}, 'key5': 'print', 'key6': 100}

# we can also add a key and value pair in following fashion
# x[new_key] = value

x['xyz'] = {'abc':123}

And the output is:

{'key1': <function print>,
 'key2': [1, 2, 3],
 'key3': (11, 12, 13),
 'key4': {4, 5, 6},
 'key5': 'print',
 'key6': 100,
 'xyz': {'abc': 123}}

x['xyz'] = {'def':456} # overriding the old key and value
x

And the output is:

{'key1': <function print>,
 'key2': [1, 2, 3],
 'key3': (11, 12, 13),
 'key4': {4, 5, 6},
 'key5': 'print',
 'key6': 100,
 'xyz': {'def': 456}}

get()

• we use get() to fetch a value

x.get('xyz')

And the output is:
{'def': 456}

# we can also fetch a value in following fashion
x['xyz']

{'def': 456}

keys, values, items

• keys(), values() and items() returns a dict_keys, dict_values, dict_items objects.
• These objects have the keys, values and key & value and tuples of’key and value pairs as elements respectively
• we need to type cast these objects in lists or tuple to see them

a,b,c = x.keys(), x.values(), x.items()

print(a,b,c, sep ='\n')
print()
print(type(a), type(b), type(c))

And the output is:

dict_keys(['key1', 'key2', 'key3', 'key4', 'key5', 'key6', 'xyz'])
dict_values([<built-in function print>, [1, 2, 3], (11, 12, 13), {4, 5, 6}, 'print', 100, {'def': 456}])
dict_items([('key1', <built-in function print>), ('key2', [1, 2, 3]), ('key3', (11, 12, 13)), ('key4', {4, 5, 6}), ('key5', 'print'), ('key6', 100), ('xyz', {'def': 456})])

<class 'dict_keys'> <class 'dict_values'> <class 'dict_items'>

pop()

• pop takes exactly one input/arg i.e the key
• we can capture this popped ‘value’ in a var for further use

bb = x.pop('key6')
print(bb)

100

x

And the output is:

{'key1': <function print>,
 'key2': [1, 2, 3],
 'key3': (11, 12, 13),
 'key4': {4, 5, 6},
 'key5': 'print'}

popitem()

• popitem() will pop the last key & value pair if no input is given
• if an key is given as input, it will pop that key and value pair as tuple

bb = x.popitem()
print(bb)
x

('key5', 'print')
{'key1': <function print>,
 'key2': [1, 2, 3],
 'key3': (11, 12, 13),
 'key4': {4, 5,6}}

fromkeys()

• it will create a new dict from the keys of given dict, values by default will be none
• fromkeys() is a class method — -> we will see this in oops

list(x.keys())

['key1', 'key2', 'key3', 'key4']

y = dict.fromkeys(x)
print(y)

{'key1': None, 'key2': None, 'key3': None, 'key4': None}

setdefault()

• The setdefault() method returns the value of the item with the specified key.
• we use to set the default value, if it is not present in dictionary.
• if the key does not exist, then it returns ‘None’
• if we give the default key, value pairs then it display the default value, when the defaultkey is called

x = {1:'milk',
     2:'tea powder',
     3:'sugar',
      4:[1,2,3],
      5:{4,5,6}}
print(x)
y = x.setdefault("6")
print(y)

And the output is:

{1: 'milk', 2: 'tea powder', 3: 'sugar', 4: [1, 2, 3], 5: {4, 5, 6}}
None

# to pull a value without a key or when there is no key
x.get('hello')
x