Master Python’s Core Data Structure Methods: Strings, Lists, Dictionaries, and Sets

String Methods

Note: strings are immutable and cannot be assigned via slicing.

1. find(str, start) returns the lowest index of the substring or -1 if not found; case‑sensitive.

>> title = 'Python is the best language in the world'
>>> title.find('python')
-1
>>> title.find('the', 10)
10
>>> title.find('the', 11)
31

2. join(iterable) concatenates the elements of an iterable using the string as a separator; all elements must be strings.

>> '-'.join({'name':'chenyy','age':18})
'name-age'

3. split(str, num) splits a string by the given separator into a list; num limits the number of splits. If no separator is provided, any whitespace is used.

>> s = 'to be or not to be'
>>> s.split(' ')
['to', 'be', 'or', 'not', 'to', 'be']

4. lower() returns a copy of the string converted to lowercase.

>> 'Hello World'.lower()
'hello world'

5. title() capitalizes the first character of each word.

>> 'hello world'.title()
'Hello World'

6. replace(old, new) returns a new string where all occurrences of old are replaced by new .

>> 'this is test'.replace('test','demo')
'this is demo'

7. strip([chars]) removes the specified characters from both ends of the string.

>> '-*- coding:utf-8 -*-'.strip('-*-')
' coding:utf-8 '

8. swapcase() swaps the case of each character in the string.

>> "I love Python".swapcase()
'i LOVE pYTHON'

9. isalnum() checks whether the string consists only of alphanumeric characters.

>> 'python 666'.isalnum()
False
>>> 'python'.isalnum()
True
>>> '666'.isalnum()
True

10. isalpha() checks whether the string consists only of alphabetic characters.

>> 'python123'.isalpha()
False
>>> 'python'.isalpha()
True

11. isdigit() checks whether the string consists only of digits.

>> '123'.isdigit()
True
>>> 'python123'.isdigit()
False

12. startswith(str, start, end) returns True if the string begins with the specified prefix.

>> 'to be or not to be'.startswith('to')
True

13. endswith(str, start, end) returns True if the string ends with the specified suffix.

>> 'to be or not to be'.endswith('be')
True

List Methods

1. append(value) adds an element to the end of the list (in‑place).

>> lst = [1, 2, 3]
>>> lst.append(4)
>>> lst
[1, 2, 3, 4]

2. count(value) returns the number of occurrences of value in the list (also works for tuples and strings).

>> x = [[1, 2], 1, 1, [1, [1, 2]]]
>>> x.count(1)
2
>>> x.count([1, 2])
1

3. extend(iterable) appends each element from the iterable to the list (modifies the original list).

>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> a.extend(b)
>>> a
[1, 2, 3, 4, 5, 6]

Equivalent effect can be achieved with slice assignment:

>> a = [1, 2, 3]
>>> b = [4, 5, 6]
>>> a[len(a):] = b
>>> a
[1, 2, 3, 4, 5, 6]

4. index(value) returns the index of the first occurrence of value ; raises ValueError if not found.

>> lst = [1, 2, 3]
>>> lst.index(2)
1

5. insert(index, value) inserts value at the given position; if the index is out of range, the value is appended.

>> lst = [1, 3, 4]
>>> lst.insert(1, 'two')
>>> lst
[1, 'two', 3, 4]

6. pop([index]) removes and returns the element at index (default last); the only list method that both mutates and returns a value.

>> lst = [1, 2, 3, 4, 5]
>>> lst.pop()
5
>>> lst.pop(1)
2

7. remove(value) deletes the first matching element; does not return a value.

>> lst = ['to', 'be', 'or', 'not', 'to', 'be']
>>> lst.remove('to')
>>> lst
['be', 'or', 'not', 'to', 'be']

8. reverse() reverses the list in place.

>> lst = [1, 2, 3, 4]
>>> lst.reverse()
>>> lst
[4, 3, 2, 1]

9. sort() sorts the list in place; all elements must be of comparable types.

>> lst = ['one', 'two', 'three', 'four']
>>> id(lst)
1652726159944
>>> lst.sort()
>>> lst
['four', 'one', 'three', 'two']
>>> id(lst)
1652726159944

Dictionary Methods

1. clear() removes all items from the dictionary (in‑place).

>> d = {'name':'chenyy','age':18}
>>> id(d)
1652725275168
>>> d.clear()
>>> d
{}
>>> id(d)
1652725275168

2. copy() returns a shallow copy of the dictionary.

>> d = {'name':'chenyy','age':18}
>>> d1 = d.copy()
>>> id(d)
1652726134608
>>> id(d1)
1652725275168
>>> d1
{'name':'chenyy','age':18}

3. fromkeys(iterable, default=None) creates a new dictionary with keys from the iterable and all values set to default (default is None).

>> dict().fromkeys(['name','age'])
{'name': None, 'age': None}
>>> dict.fromkeys(['name','age'],'null')
{'name': 'null', 'age': 'null'}

4. get(key, default=None) returns the value for key if present; otherwise returns default (default None).

>> d = {'name':'chenyy','age':18}
>>> d.get('sex')
>>> d.get('sex','boy')
'boy'

5. items() returns a view of the dictionary’s key‑value pairs.

>> d.items()
dict_items([('name', 'chenyy'), ('age', 18)])

6. keys() returns a view of the dictionary’s keys.

>> d.keys()
dict_keys(['name', 'age'])

7. values() returns a view of the dictionary’s values.

>> d.values()
dict_values(['chenyy', 18])

8. pop(key) removes the specified key and returns its value.

>> d = {'name':'chenyy','age':18}
>>> d.pop('age')
18
>>> d
{'name': 'chenyy'}

9. popitem() removes and returns an arbitrary (key, value) pair as a tuple.

>> d = {'name':'chenyy','age':18,'sex':'boy'}
>>> d.popitem()
('sex', 'boy')

10. setdefault(key, default=None) returns the value of key if it exists; otherwise inserts key with default and returns default .

>> d = {}
>>> d.setdefault('name','chenyy')
'chenyy'
>>> d
{'name': 'chenyy'}

11. update(iterable) merges another mapping or iterable of key‑value pairs into the dictionary; existing keys are overwritten.

>> d = {'sex':'boy'}
>>> id(d)
1782678051288
>>> d.update([('name','chenyy'),('age',18)])
>>> d
{'sex': 'boy', 'name': 'chenyy', 'age': 18}
>>> id(d)
1782678051288

Set Methods

Note: frozenset() creates an immutable set; sets cannot contain mutable objects such as lists or dictionaries.

Non‑operator forms ( set.union(), set.intersection(), etc.) accept any iterable, while operator forms ( |, &, etc.) require another set.

1. difference ( - ) returns elements present only in the first set.

>> set1 = {1,2,3,4,5,6}
>>> set2 = {4,5,6,7,8,9}
>>> set1 - set2
{1, 2, 3}

2. intersection ( & ) returns common elements of both sets.

>> set1 & set2
{4, 5, 6}

3. union ( | ) returns all unique elements from both sets.

>> set1 | set2
{1, 2, 3, 4, 5, 6, 7, 8, 9}

4. symmetric_difference ( ^ ) returns elements that are in either set but not in both.

>> s1 = {1,2,3}
>>> s2 = {2,3,4}
>>> s1 ^ s2
{1, 4}

5. pop() removes and returns an arbitrary element; raises KeyError if the set is empty.

6. add(value) adds an element to the set.

7. remove(value) removes the specified element; raises KeyError if not present.

8. discard(value) removes the element if it exists; does nothing otherwise.

9. issubset(other) ( <= ) checks whether all elements of the set are contained in other .

>> s2 <= s1
True

10. issuperset(other) ( >= ) checks whether the set contains all elements of other .

>> s1.issuperset(s2)
True

11. update(other) ( |= ) adds all elements from other to the set (in‑place).

>> set1 |= set2
>>> set1
{1, 2, 3, 4, 5, 6}

12. intersection_update(other) ( &= ) retains only elements found in both sets (in‑place).

>> set1 &= set2
>>> set1
{4, 5, 6}

13. difference_update(other) ( -= ) removes all elements of other from the set (in‑place).

14. symmetric_difference_update(other) ( ^= ) updates the set to the symmetric difference of itself and other (in‑place).