Hope you're now pretty comfortable with python basics after all we've gone through variables, data types, operators, and strings manipulation in the last sessions. You may want to check them if you haven't yet.
List is one of the most common and versatile data structures we use in python when it comes to storing a group of items or values.
You can construct a list in python simply as a list of comma-separated values (items) in between square brackets.
Like this:
numbers = [1, 2, 3, 4, 5, 6, 7]Elements in a list can be of different data types and can contain another (nested) lists as well.
misc_list = [1, 'foo', '2', 'bar', [5, 6, 7]]Like strings, lists are sequences too and thus support all the operations that sequences in python support.
For example:
>>> numbers = [1, 2, 3, 4, 5, 6, 7]
>>> len(numbers)
7
>>> min(numbers)
1
>>> max(numbers)
7
>>> numbers.count(5)
1
>>> 7 in numbers
True
>>> 8 in numbers
False
>>> 0 not in numbers
TrueLists do support even concatenation just the way strings do with the + operator.
>>> numbers + [8, 9, 10]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]Check the common operations that lists support just like any other sequence types.
Unlike strings, lists are mutable data types; thus it's possible to mutate or change the list like this:
names = ['John Doe', 'Jane Doe', 'Johnny Turk']
# Change the first name in the list
names[0] = 'Foo Bar'
print('Names now:', names)
# Append some more names
names.append('Molly Mormon')
names.append('Joe Bloggs')
print('Names finally:', names)
print('Last name in the list: %s' % names[-1])
# You can join lists using str.join() method
joined_names = '\n'.join(names)
print('\nList of names:')
print(joined_names)The list object support the following methods.
| Method | Description |
|---|---|
| append(x) | Add an item to the end of the list. |
| insert(i, x) | Insert an item at a given position. |
| remove(x) | Remove an item from the list whose value equals to x. An error is thrown if the value is not found in the list. |
| pop([i]) | Remove the item at ith position in the list, and return it. If index i is not, the last item is removed and returned |
| clear() | Remove all items from the list. |
| index(x[, start[, end]]) | Return the first index whose value is x. Throws an error if the value is not found in the list. |
| count(x) | Count the number of times the value x appears in the list. |
| sort(x) | Sort the items of the list. |
| reverse() | Reverse the items of the list. |
| copy() | Return a shallow copy of the list. |
Check the official docs for lists more more information.
Let's try these methods as well.
>>> names.insert(0, 'Mark Joe')
>>> names
['Mark Joe', 'Foo Bar', 'John Doe', 'Jane Doe', 'Johnny Turk', 'Molly Mormon', 'Joe Bloggs']
>>> names.remove('Foo Bar')
>>> names
['Mark Joe', 'John Doe', 'Jane Doe', 'Johnny Turk', 'Molly Mormon', 'Joe Bloggs']
>>> names.pop()
'Joe Bloggs'
>>> names.pop(0)
'Mark Joe'
>>> names
['John Doe', 'Jane Doe', 'Johnny Turk', 'Molly Mormon']
>>> names.sort()
>>> names
['Jane Doe', 'John Doe', 'Johnny Turk', 'Molly Mormon']
>>> names.index('Molly Mormon')
3List comprehension is a pythonic way of creating lists in a concise manner based upon the results of some operations or certain conditions. List comprehensions is one of the most popular features of python lists.
# Create a list of squares of numbers upto 10
squares = [x**2 for x in range(10)]
print('Squares:', squares)
# You can create lists using existing lists.
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_numbers = [x for x in numbers if x % 2 ==0]
odd_numbers = [x for x in numbers if x % 2 !=0]
print('Numbers:', numbers)
print('Even numbers:', even_numbers)
print('Odd numbers:', odd_numbers)# You can even create new lists by processing existing lists.
words = ['this', 'is', 'just', 'a', 'test']
capitalized_words = [x.capitalize() for x in words]
print('Words:', words)
print('Capitalized Words:', capitalized_words)
# Can use it for filtering the list items as well.
words = ['hello', 'world', 'foo', 'bar', 'test', 'python', 'is', 'awesome']
short_words = [x for x in words if len(x) <= 3]
other_words = [x for x in words if x not in short_words]
words_with_e = [x for x in words if x.count('e') >= 1]
print('Words:', words)
print('Short Words:', short_words)
print('Other Words:', other_words)
print('Words with "e":', words_with_e)Looping through list is really simple in python. You can use for loop for looping through lists.
names = ['John Doe', 'Jane Doe', 'Johnny Turk']
print('Names:')
for name in names:
print(' - %s' % name)You might often want to check if the list is empty. Usually the list is dynamically generated. You can do that by checking no. of items in the list is zero or not.
my_list = []
if len(my_list) == 0:
print('No items on the list.')
else:
print(my_list)Note: We'll discuss more about loops and conditions in the upcoming sessions.
Another most common data structure used in python is a dictionary. The main difference between sequence types like strings & lists and dictionaries is that sequences are indexed by range of numeric indexes but dictionaries are indexed by keys.
Any immutable data type can be used as keys in dictionaries, usually they are strings and numbers.
You can always think of a dictionary as a set of key value pairs.
You can create a dictionary like this:
user_info = {
'name': 'Kabir Baidhya',
'email': '[email protected]',
'address': 'Kathmandu, Nepal'
}
# Accessing key from a dict is just similar to lists.
print('Name: %s' % user_info['name'])
print('Email: %s' % user_info['email'])
print('Address: %s' % user_info['address'])Since dictionaries are mutable types you can mutate them just like lists.
user_info['name'] = 'Kabir'
user_info['email'] = user_info['email'].replace('@gmail.com', '[email protected]')
# If the key doesn't already exists it would create a new key-value pair.
user_info['dob'] = '1992-07-30'
# And you can store any type of values inside a dict. Even lists.
user_info['hobbies'] = ['Music', 'Travelling', 'Coding']
print(user_info)You can create a list of dictionaries if you want to store similar information about multiple entities.
# This is a list of dictionaries.
data = [
{
'name': 'Kabir Baidhya',
'email': '[email protected]'
},
{
'name': 'John Doe',
'email': '[email protected]'
}
]
# Print information from the dictionary
print('Name: %s' % data[1]['name'])
print('Email: %s' % data[1]['email'])Following are some of the common dictionary operations.
| Operation | Description |
|---|---|
| len(d) | Return the number of items in the dictionary d. |
| d[key] | Access/Return the item of dictionary identified by key key. An error is thrown if key is not found in the dictionary. |
| d[key] = value | Set a value in the dictionary identified by key. |
| del d[key] | Remove the item with key key from the dictionary. An error is thrown if key does not exists |
| key in d | Check if the key exists in the dictionary. Return True or False. |
| key not in d | Check if the key doesn't exist in the dictionary i.e just the opposite of key in d. Return True or False. |
| clear() | Remove all the items from the dictionary. |
| copy() | Return a shallow copy of the dictionary. |
Read more about dictionaries here.
- Store a list of at least 20 words in a list. Ask the user to enter a string(partial) and print out the list of suggestions based on whether or not the word starts with the string entered. Note: the suggestion should be case-insensitive. (Hint: List comprehension)
- Store a list of user information in a list of dictionaries. Each user's information would contain: first & last name, email and address. Ask the user to input an email address. Print the first user's information found by that email address. Print "Email not found" message if user with email not found. (Hint: List comprehension)
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