Dictionary

Each key is separated from its value by a colon (:), the items are separated by commas, and the whole thing is enclosed in curly braces. It is implemented as a hash table and hence most operations such as insertion, deletion and query take O(1) time.

clear()

The syntax of clear() is dict.clear() The clear() method removes all items from the dictionary. It doesnt take any parameters and doesnt return any value( returns None).

Example:

d = {1: "one", 2: "two"}
d.clear()
print('d =', d)

Output:

d = {}

Elements can also be removed by assigning empty dictionary {} . However, there is a difference between the two if there is a variable referencing the dictionary.

Example:

d = {1: "one", 2: "two"}
d1 = d
d.clear()
print('Removing items using clear()')
print('d =', d)
print('d1 =', d1)
d = {1: "one", 2: "two"}
d1 = d
d = {}
print('Removing items by assigning {}')
print('d =', d)
print('d1 =', d1)

Output:

Removing items using clear()
d = {}
d1 = {}
Removing items by assigning {}
d = {}
d1 = {1: 'one', 2: 'two'}

copy()

The syntax of copy() is dict.copy() They copy() method returns a shallow copy of the dictionary. It doesn’t modify the original dictionary. Complexity: O(n)

Example:

original = {1:'one', 2:'two'}
new = original.copy()
print('Orignal: ', original)
print('New: ', new)

Output:

Orignal:  {1: 'one', 2: 'two'}
New:  {1: 'one', 2: 'two'}

fromkeys()

The syntax of fromkeys() method is: dictionary.fromkeys(sequence[, value]) The fromkeys() method creates a new dictionary from the given sequence of elements with a value provided by the user. The fromkeys() method takes two parameters:

sequence - sequence of elements which is to be used as keys for the new dictionary.
value (Optional) - value which is set to each each element of the dictionary.

The fromkeys() method returns a new dictionary with the given sequence of elements as the keys of the dictionary. If the value argument is set, each element of the newly created dictionary is set to the provided value.

Example:

keys = {'a', 'e', 'i', 'o', 'u' }
value = 'vowel'
vowels = dict.fromkeys(keys, value)
print(vowels)

Output:

{'a': 'vowel', 'u': 'vowel', 'o': 'vowel', 'e': 'vowel', 'i': 'vowel'}

get()

The syntax of get() is: dict.get(key[, value]) The get() method returns the value for the specified key if key is in dictionary. The get() method takes maximum of two parameters:

key - key to be searched in the dictionary.
value (optional) - Value to be returned if the key is not found. The default value is None.

The get() method returns: the value for the specified key if key is in dictionary.

None if the key is not found and value is not specified.
value if the key is not found and value is specified.

Example:

person = {'name': 'Barney', 'age': 21}
print('Name: ', person.get('name'))
print('Age: ', person.get('age'))
print('Salary: ', person.get('salary'))
print('Salary: ', person.get('salary', 0.0))

Output:

Name:  Barney
Age:  21
Salary:  None
Salary:  0.0

items()

The syntax of items() method is: dictionary.items() The items() method returns a view object that displays a list of dictionary’s (key, value) tuple pairs.

Example:

abc = { 'One': 1, 'Two': 2, 'Three': 3 }
print(abc.items())

Output:

dict_items([('One', 1), ('Two', 2), ('Three', 3)])

keys()

The syntax of keys() is: dict.keys() The keys() method returns a view object that displays a list of all the keys in the dictionary. When the dictionary is changed, the view object also reflect these changes.

Example:

person = {'name': 'Abc', 'age': 21, 'salary': 1200.0}
print(person.keys())
empty_dict = {}
print(empty_dict.keys())

Output:

dict_keys(['name', 'age', 'salary'])
dict_keys([])

pop()

The syntax of pop() method is dictionary.pop(key[, default]) The pop() method removes and returns an element from a dictionary having the given key. The pop() method takes two parameters:

key - key which is to be searched for removal
default - value which is to be returned when the key is not in the dictionary

The pop() method returns:

If key is found - removed/popped element from the dictionary
If key is not found - value specified as the second argument (default)
If key is not found and default argument is not specified - KeyError exception is raised

Example:

sales = { 'apple': 2, 'orange': 3, 'grapes': 4 }
element = sales.pop('apple')
print('The popped element is:', element)
print('The dictionary is:', sales)

Output:

The popped element is: 2
The dictionary is: {'orange': 3, 'grapes': 4}

popitem()

The syntax of popitem() is: dict.popitem() The popitem() returns and removes an arbitrary element (key, value) pair from the dictionary. The popitem()

returns an arbitrary element (key, value) pair from the dictionary.
removes an arbitrary element (the same element which is returned) from the dictionary.

Example:

person = {'name': 'Abc', 'age': 21, 'salary': 1200.0}
result = person.popitem()
print('person = ',person)
print('Return Value = ',result)

Output:

person =  {'name': 'Abc', 'salary': 1200.0}
result =  ('age', 21)

The popitem() raises a KeyError error if the dictionary is empty.

setdefault()

The syntax of setdefault() is: dict.setdefault(key[, default_value]) The setdefault() method returns the value of a key (if the key is in dictionary). If not, it inserts key with a value to the dictionary. The setdefault() takes maximum of two parameters:

key - key to be searched in the dictionary
default_value (optional) - key with a value default_value is inserted to the dictionary if key is not in the dictionary. If not provided, the default_value will be None.

The setdefault() returns:

value of the key if it is in the dictionary.
None if key is not in the dictionary and default_value is not specified.
default_value if key is not in the dictionary and default_value is specified.

Example:

person = {'name': 'Abc', 'age': 21}
age = person.setdefault('age')
print('person = ',person)
print('Age = ',age)

Output:

person =  {'name': 'Abc', 'age': 21}
Age =  21

update()

The syntax of update() is: dict.update([other]) The update() method updates the dictionary with the elements from another dictionary object or from an iterable of key/value pairs. It adds element(s) to the dictionary if the key is not in the dictionary. If the key is in the dictionary, it updates the key with the new value. The update() method takes either a dictionary or an iterable object of key/value pairs (generally tuples). If update() is called without passing parameters, the dictionary remains unchanged.

Example:

d = {1: "one", 2: "three"}
d1 = {2: "two"}

d.update(d1)
print(d)
d1 = {3: "three"}

d.update(d1)
print(d)

Output: ```{1: ‘one’, 2: ‘two’} {1: ‘one’, 2: ‘two’, 3: ‘three’}

### values()
The syntax of values() is:
`dictionary.values()`
The values() method returns a view object that displays a list of all the values in the dictionary.

Example:
```python
sales = { 'apple': 2, 'orange': 3, 'grapes': 4 }
print(sales.values())

Output:

dict_values([2, 4, 3])

all()

The syntax of all() method is: all(iterable) The all() method takes a single parameter:

iterable - any iterable (list, tuple, dictionary, etc.) which contains the elements.

If all keys (not values) are true or the dictionary is empty, all() returns True. Else, it returns false for all other cases.

Example:

s = {0: 'False', 1: 'False'}
print(all(s))
s = {1: 'True', 2: 'True'}
print(all(s))
s = {1: 'True', False: 0}
print(all(s))
s = {}
print(all(s))

s = {'0': 'True'}
print(all(s))

Output:

False
True
False
True
True

any()

The syntax of any() is: any(iterable) If all keys (not values) are false, any() returns False. If at least one key is true, any() returns True.

Example:

d = {0: 'False'}
print(any(d))
d = {0: 'False', 1: 'True'}
print(any(d))
d = {0: 'False', False: 0}
print(any(d))
d = {}
print(any(d))

d = {'0': 'False'}
print(any(d))

Output:

False
True
False
False
True

len()

The syntax of len() is: len(s) The len() function returns the number of items (length) of an object. Failing to pass an argument or passing an invalid argument will raise a TypeError exception.

Example:

dict = {'Name': 'Manni', 'Age': 7, 'Class': 'First'}
print ("Length : %d" % len (dict))

Output:

Length : 3

sorted()

The syntax of sorted() method is: sorted(iterable[, key][, reverse]) sorted() takes two three parameters:

iterable - collection (dictionary) or any iterator
reverse (Optional) - If true, the sorted list is reversed (or sorted in Descending order)
key (Optional) - function that serves as a key for the sort comparison

sorted() method returns a sorted list from the given iterable.

Example:

pyDict = {'e': 1, 'a': 2, 'u': 3, 'o': 4, 'i': 5}
print(sorted(pyDict, reverse=True))

Output:

['u', 'o', 'i', 'e', 'a']

cmp()

The syntax of cmp() method is: cmp(dict1, dict2) The method cmp() compares two dictionaries based on key and values. This method returns 0 if both dictionaries are equal, -1 if dict1 < dict2 and 1 if dict1 > dict2. Example:

dict1 = {'Name': 'Zen', 'Age': 7};
dict2 = {'Name': 'Mac', 'Age': 27};
print "Return Value : %d" %  cmp (dict1, dict2)

Output:

Return Value : -1

standard-library-in-x

Notes and readings for STL workshop

Dictionary

clear()

copy()

fromkeys()

get()

items()

keys()

pop()

popitem()

setdefault()

update()

all()

any()

len()

sorted()

cmp()