You still have to take notes when you study. One is to deepen your impression of learning, and the other is to facilitate future review. I would like to thank CSDN for providing this platform.

He is not a pure professional. He mainly studied C language in school. He has almost forgotten it now. He took the opportunity to learn Python to improve his programming ability.

catalogue

1: Python installation and execution

2: Identifiers and keywords in Python

3: Python expressions, indents, and comments

3.1 expression

3.2 indent

3.3 notes

4: Variables and literals in Python

4.1 variables

4.2 face value

Summary:

1: Python installation and execution

Only methods under Windows are provided here. Please search for other operating systems by yourself.

Directly search the official Python website on the web page and download it. What I download here is the latest 3.9.6.

After downloading and installing, there will be several things. What we need is IDLE (python 3.x,)

python programming is different from C language. python is divided into two programming modes: 1 Interactive programming; 2. Document type.

After entering IDLE, interactive programming is displayed. As shown in the figure below.

Python 3.9.5 (default, May  4 2021, 03:36:27) 
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> # Execute single line code
>>> print("Hello")
Hello
>>>
>>> # Entering the \ symbol at the end of the statement can perform line feed and execute multiple lines of code
>>> if True: \
... print(True)
... 
True
>>> 

In this way, python will follow your footsteps to execute your statements, so that you can clearly see the execution results of the statements you write.

Another is to click file and New file in the upper left corner after entering IDLE. Open a file, write your code in it, and then save it. Remember when you save it, don't you py suffix. This is file programming.

I prefer the second method.

2: Identifiers and keywords in Python

The following identifiers are reserved words or keywords of the language and cannot be used as ordinary identifiers. Keywords must be spelled exactly as listed here.

False      await      else       import     pass
None       break      except     in         raise
True       class      finally    is         return
and        continue   for        lambda     try
as         def        from       nonlocal   while
assert     del        global     not        with
async      elif       if         or         yield

These keywords are very important. You need to remember the usage of these keywords! Remember! Remember!

In addition to keywords, some identifier classes have special meanings. The naming pattern of these identifier classes is the beginning and end of the following underlined characters: (these can be understood, do not need to be understood)

• _*

  It will not be imported by "from module import *. Special identifier_ It is used to store the latest evaluation result in the interactive interpreter; It is saved in builtins Module. When not in interactive mode_ It has no special meaning and is not predefined. See import statement  . Annotation_ As a name, it is often used to connect international text; See gettext Module documentation for details on this engagement.

• __*__

  The system defined name is informally called "dunder". These names are defined by the interpreter and its implementation, including the standard library. More such names will be defined in future Python releases. In any case, any} does not comply with the requirements explicitly indicated in the document__*__ Name usage can lead to an error without warning.

• __*

  Private name of the class. When used in a class definition, this name is overridden in a mixed form to avoid name conflicts between the "private" properties of the base and derived classes. See Identifier (name) .

3: Python expressions, indents, and comments

3.1 expression

Expression is the basis of any programming language. In short, a minimum expression with complete functions is an expression.

Example: simplest expression

print("Hello World")

Example: incomplete expression

for v in [1,2,3]:

#The for loop statement does not contain a loop body

After modification

for v in [1,2,3]:
    print(v)

Multiline expression

By default, Python expressions do not support line feed writing. Example:

# Writing error
ab = "a" +
"b"

We can use the \ symbol to realize line feed, which can be understood as telling the interpreter that the expression is not over. Example:

# Correct writing
ab = "a" + \
"b"

What do you call an interpreter? The interpreter is similar to the compiler. It is to compile the code we write into the code well-known to the computer. What's the difference between an interpreter and a compiler? A: 1. An interpreter is a program that directly executes instructions written in a programming language, and a compiler is a program that converts the source code into a low-level language; 2. The compiler generates an independent program, and the interpreted program always needs the interpreter to run. If you still don't understand, you can find it yourself.

In addition, the expressions in the three symbols (), [], {} (see the difference between the three symbols below) support line feed by default. Example:

# () is represented as a whole, and () will increase the calculation priority in mathematical calculation
str = (
    "a" +
    "b" +
    "c"
)

# [] represents a List structure (List)
arr = [
        "a",
        "b",
        "c",
]

# {} represents a dictionary structure
tel = {
    'jack': 4098,
    'sape': 4139
}

In Python; The symbol default expression does not need to end with a semicolon (this is also a big difference from the C language). If you need to write two expressions on the same line, you can separate them with a semicolon. Example:

print("Hello"); print("World")

3.2 indent

Most programming languages use {} symbols to define code blocks. Examples:

// PHP language code
if (true) {
    echo "Hello";
}

// C language code
#include 
int main() {
    if (1 > 0) {
		printf("Hello");
	}
	return 0;
}

Use indentation instead of {} to define code blocks in Python (you can't arbitrarily indent the first line!), example:

if True:
    print("Hello World")

# Write an expression on a blank line

Indent correctly, indenting your code block with 4 spaces. Python code must be indented strictly, or the Python interpreter will throw out errors:

IndentationError: expected an indented block

3.3 notes

Comments are used to explain the function of a piece of code. A single line comment can start with #. Python uses three double quotes' ', or three single quotes'', for multiline comments.

python document string

Let's take a look at an example:

# Specify a function that does not contain a document string
def fn_str():
    print("fn_str")

# Use__ doc__  Print help for abc
print(fn_str.__doc__)
# "None" will be output

# Add document string for function
def fn_str():
    """This is a function"""
    print("fn_str")

print(fn_str.__doc__)
# The output "this is a function" is displayed

The document string is used to add document information for functions or classes. In addition to printing with doc, it can also be viewed with the help() function.  

4: Variables and literals in Python

4.1 variables

Variable is one of the most important elements of programming language

Let's start with an example:

foo = "a"
# Foo is a variable, and the value of foo is "a"

# It is called a variable because the value of the variable can be changed
foo = "b"
print(foo) # Output b

Variable is the carrier of data. Python can store various data types.

Python uses the = sign to declare variables

numberVar = 123 # Declare a variable named numberVar whose value is a number
floatVar = 123 # Declare a variable named numberVar whose value is a number
booleVar = True # Declare a variable named numberVar whose value is a number
noneVar = True # Declare a variable named numberVar whose value is a number
listVar = [1,2,3,4,5] # Declare a variable named numberVar whose value is a number

# You can also "store" a function in a variable 👇
def func():
    print("this a function")

funcVar = func

funcVar()
# Output "this a function"

4.2 face value

The literal value is the specific value of the text and cannot be modified. It may seem awkward: the literal value of 10 is 10.

Example:

print(10) # Print the literal value of 10 and output 10
print("abc") # Print the literal value of abc and output abc

# If you try to modify the literal value, you will receive an error explained by Python
"abc" = 111
#Output: error: syntax error: cannot assign to literal

Summary:

1, This section describes how to install Python on different systems and how to run the Python interpreter and execute Python code.

main points:

1. Using python -c command [arg] Run Python code in mode
2. Using python -m module [arg] Running Python modules in mode
3. Executing Python code in interactive mode

2, Please memorize the keywords.

3, This section describes how Python writes but line and multiline expressions, how to indent, how to write comments, and document strings

main points:

1. Single line expression
2. Multiline expressions: \, (), {}, []
3. Notes: #, '', ''“
4. Document string: "" Docstrings "" "

4, In this section, readers can follow the code example to practice how to declare the ， variable ， and what is the ， literal.