Common methods: Lists, tuples, dictionaries and strings can be used together.

Python built-in functions

Listing of built-in functions

Note: Strings have the following rules: "0"< "A"< "a"

Use of built-in functions

len() calculates the number of elements in the container;

Del uses keywords and functions, and the results are the same in both ways; del variable; del (variable);

list = [1, 2,4]
del list[1]
list
[1, 4]
del(list[0])
list
[4]
del(list)
# After deleting the list, an error will be reported and undefined

max and min functions

test_str = "qeqfeijovodmbahidkolmc"
max(test_str)
'v'
min(test_str)
'a'
# If it's a dictionary, it's only for key comparisons
test_list = [3, 9, 0, 1]
max(test_list)
9
min(test_list)
tset_dict = {"a": "3", "b": "1", "c": "2"}
max(tset_dict)
'c'
min(tset_dict)
'a'

In Python 3, the cmp comparison operator is cancelled, but we can directly compare by comparison operator <>.
Numbers can be compared, strings can be compared, tuples, lists can be compared, but dictionaries can not compare sizes.

Section

• Slices use index values to limit ranges and cut small strings from a large string.
• Strings, lists, tuples can be sliced because they are ordered, but dictionaries cannot slice because dictionaries are disordered.

Examples of slices are as follows:

t_list = [3,1,2,5,7]
t_list[0:3]
[3, 1, 2]
t_tuple = (1,2,3,4,5)
t_tuple[0:3]
(1, 2, 3)

operator

Operator listing

• When in operates on a dictionary, it judges the key of the dictionary.
• In and not in are called member operators

Operator usage

* It can be used for list tuples, but not dictionaries, because dictionary key s must be unique;

[1,2]*5
[1, 2, 1, 2, 1, 2, 1, 2, 1, 2]
(1,2)*5
(1, 2, 1, 2, 1, 2, 1, 2, 1, 2)

+ Operators represent merges and can be used to merge lists, tuples and strings; different types cannot be merged, such as tuples and lists;

"hello" + "world"
'helloworld'
(1,2) + (3,4)
(1, 2, 3, 4)
[1,2] + [3,4]
[1, 2, 3, 4]

Note that when we use + merge lists, we generate a new list

The difference between append and extend methods is that extend combines a specified list parameter into a list, i. e. scattered addition, while append combines a specified list as a whole element into a list.
Using append and extend is to extend elements on the original list.

Use of in and not in

• Inclusion and non-inclusion;
• It can be used for strings, lists, tuples, dictionaries; however, it should be noted that in the dictionary, in only checks for key s in the dictionary.

The use of in and not in is as follows:

"a" in "asdfg"
True
"a" not in "asdfg"
False
2 in (1,2,3,4)
True
2 not in [1,2,3,4]
False
"name" in {"name": "zhangsan"}
True
"zhangsan" in {"name": "zhangsan"}
False

Complete for loop

Complete for loop grammar

for xx in Collection:
    xxx
else:
    xxx

This is the grammatical structure of the complete for loop.

• As long as the for loop is not interrupted by break, the code in else will execute after the for loop ends.
• If the for loop is interrupted, subsequent code outside the for loop is executed directly

Complete for loop demonstration

students = [{"name": "Hanxin"}, {"name": "Li Bai"}, {"name": "Liu Bei"}]

for stu in students:
    print(stu)
else:
    print("I am for At the end of the loop traversal else Sentence")
print("for The cycle is over")

# {name':'hanxin'}
# {name':'Li Bai'}
# {name':'Liu Bei'}
# I'm the else statement after the for loop traversal
# The for loop is over

break interrupts for loop demonstration

students = [{"name": "Hanxin"}, {"name": "Li Bai"}, {"name": "Liu Bei"}]

for stu in students:
    print(stu)
    if stu["name"] == "Li Bai":
        print("Li Bai, your mother called you home for dinner.")
        break
else:
    print("I am for At the end of the loop traversal else Sentence")
print("for The cycle is over")

# {name':'hanxin'}
# {name':'Li Bai'}
# Li Bai, your mother called you home for dinner.
# The for loop is over

Application scenarios for else

• When iterating over nested data types, for example, a list contains multiple dictionaries

Requirement: Need to determine whether a specified value exists in a dictionary
If it exists, prompt and exit the loop
If it does not exist, after the whole cycle is over, we hope to get a unified reminder.

Find out the situation

students = [{"name": "Hanxin"}, {"name": "Li Bai"}, {"name": "Liu Bei"}]

find_stu = "Li Bai"

for stu in students:
    print("Looking for...", stu["name"])
    if stu["name"] == find_stu:
        print("%s，Your mother told you to go home for dinner." % find_stu)
        break
else:
    print("Excuse me, your home%s Not in this class" % find_stu)
print("for The cycle is over")

# Looking for Han Xin
# Looking for... Li Bai
# Li Bai, your mother called you home for dinner.
# The for loop is over

Not found

students = [{"name": "Hanxin"}, {"name": "Li Bai"}, {"name": "Liu Bei"}]

find_stu = "Zhang Fei"

for stu in students:
    print("Looking for...", stu["name"])
    if stu["name"] == find_stu:
        print("%s，Your mother told you to go home for dinner." % find_stu)
        break
else:
    print("Excuse me, your home%s Not in this class" % find_stu)
print("for The cycle is over")

# Looking for Han Xin
# Looking for... Li Bai
# Looking for Liu Bei
# Sorry, Zhang Fei is not in this class.
# The for loop is over