My Python Study Notes - zishu's blog

The reason for learning Python is that I want to write some scripts or programs to scrape the text, images, and other materials I want from the browser. There are actually many programs like this on GitHub, but unfortunately, I am completely unfamiliar with backend languages, and I wouldn't know how to use them if I just took them and used them directly.

Therefore, I had the idea of learning the basics of Python first. I have always heard that Liao Xuefeng's Python tutorial is well-written, so I will take this opportunity to study it carefully. I don't expect to become extremely skilled, but at least I hope to add a little brilliance to my shallow technical stack. After all, I once dreamed of becoming a full-stack developer.

After downloading and installing the Python environment, let's start learning!

Running My First Python Program#

print('hello,world')

Data Types#

In Python, there are several types of data that can be directly processed, including integers, floating-point numbers, strings, Boolean values, null values, and variables.

This is not much different from other languages. The data types are basically similar. Only variables have their own characteristics, so I will record them separately.

Variables#

Python is a dynamic language, which means that it can be changed continuously during assignment. For example:

a = 123
print(a) # Output: 123
a = 'abc'
print(a) # Output: abc

This is allowed in Python, but not in Java. Java is a static language, and if you assign a value multiple times, it will throw an error. Relatively speaking, dynamic languages are more flexible, but each has its own advantages and disadvantages.

In addition, there is a logical order in assignment. For example, when defining a variable a = 'a', Python actually takes two steps: first, it creates a string a, and then assigns this string to the variable a.

In fact, this is somewhat illogical in mathematics. The calculation x = x + 2 is not feasible in mathematics, but in computers, it first calculates x + 2, and then assigns it to the left side x. This is the logic of computers.

x = 1
x = x + 2
print(x) # Output: 3

Many computer languages are like this, such as the well-known JavaScript, and so on.

If there are multiple variables that need to be assigned to each other, they are executed line by line.

a = 1
b = a
a = 2
print(a)
print(b)

The final output results are 2 and 1, respectively.

Python supports multiple data types. Internally, any data can be seen as an "object", and variables are used in programs to point to these data objects. Assigning a value to a variable x = y means that the variable x points to the real object, which is pointed to by the variable y. Subsequent assignments to the variable y do not affect the pointer of the variable x.
Note: Python integers have no size limit, while integers in some languages have size limits based on their storage length. For example, Java limits the range of 32-bit integers to -2147483648-2147483647.
Python's floating-point numbers also have no size limit, but if they exceed a certain range, they will be directly represented as inf (infinity). --- "Liao Xuefeng's Official Website"

Strings and Encoding#

Python provides two attributes for handling encoding, ord and chr.

ord('舒')
# 33298
# Convert the string to an integer representation

chr(33298)
# '舒'
# Convert the integer representation to a string

List#

list is similar to Array in JavaScript, which is a list of multiple data. The syntax is as follows:

>>> classmates = ['a', 'b', 'c']
>>> classmates
['a', 'b', 'c']

Now we can say that the variable classmates is a list.

There is a len() function that can output the number of elements in the list.

>>> len(classmates)
>>> 3

Lists also have indexes, starting from 0, and the last index is len(classmates)-1. If you want to directly output the last element, you can use classmates[-1].

>>> classmates[-1]
>>> 'c'

Since -1 can represent the last element, can -2 be used to represent the second to last element? The answer is yes.

>>> classmates[-2]
>>> 'b'

In addition, it should be noted that the index must not exceed the range when using indexes, otherwise an error will be thrown as shown below.

>>> classmates[4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

list is a mutable ordered list, and elements can be added or deleted from it.

append: add an element to the end

>>> classmates.append('d')
>>> classmates
['a', 'b', 'c', 'd']

insert: add an element at a specified position

In this case, 1 is the index, which means inserting the element at index 1, and the following elements will be shifted accordingly.

>>> classmates.insert(1, 'd')
>>> classmates
['a', 'd', 'b', 'c']

pop: delete the last element

>>> classmates.pop()
'c' # The deleted element when outputting

>>> classmates
['a', 'b']

pop(i): delete the element at a specified position

>>> classmates.pop(1)
'b' # The deleted element when outputting

>>> classmates
['a', 'c']
>>>