日#部分 ||从头开始重新审视 Python

Day #1 - Simple Python projects

print("Hello, World!")

What is Python?

Python is a popular programming language. It was created by Guido van Rossum, and released in 1991.

It is used for:

• web development (server-side),
• software development,
• mathematics,
• system scripting.

What can Python do?

• Python can be used on a server to create web applications.
• Python can be used alongside software to create workflows.
• Python can connect to database systems. It can also read and modify files.
• Python can be used to handle big data and perform complex mathematics.
• Python can be used for rapid prototyping, or for production-ready software development.

Why Python?

• Python works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc).
• Python has a simple syntax similar to the English language.
• Python has syntax that allows developers to write programs with fewer lines than some other programming languages.
• Python runs on an interpreter system, meaning that code can be executed as soon as it is written. This means that prototyping can be very quick.
• Python can be treated in a procedural way, an object-oriented way or a functional way.

To check if you have python installed on a Windows PC, search in the start bar for Python or run the following on the Command Line (cmd.exe):

C:\\Users\\_Your Name_\>python --version

To check if you have python installed on a Linux or Mac, then on linux open the command line or on Mac open the Terminal and type:

python --version

As we learned in the previous page, Python syntax can be executed by writing directly in the Command Line:

>>> print("Hello, World!")
Hello, World!

Or by creating a python file on the server, using the .py file extension, and running it in the Command Line:

C:\Users\Your Name>python myfile.py

Creating a Comment

Comments starts with a #, and Python will ignore them:

#This is a comment
print("Hello, World!")

Creating Variables

Python has no command for declaring a variable.

A variable is created the moment you first assign a value to it.

x = 5
y = "John"
print(x)
print(y)

Variable Names

A variable can have a short name (like x and y) or a more descriptive name (age, carname, total_volume). Rules for Python variables:

• A variable name must start with a letter or the underscore character
• A variable name cannot start with a number
• A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
• Variable names are case-sensitive (age, Age and AGE are three different variables)
• A variable name cannot be any of the Python keywords.

Global Variables

x = "awesome"

def myfunc():
  print("Python is " + x)

myfunc()

x = "awesome"

def myfunc():
  x = "fantastic"
  print("Python is " + x)

myfunc()

print("Python is " + x)

x = 1    # int
y = 2.8  # float
z = 1j   # complex

#convert from int to float:
a = float(x)

#convert from float to int:
b = int(y)

#convert from int to complex:
c = complex(x)

print(a)
print(b)
print(c)

print(type(a))
print(type(b))
print(type(c))

Random Number

import random

print(random.randrange(1, 10))

_An interesting example of Slicing:
_

explain b = "Hello, World!"
print(b[-5:-2])

1. Variable Assignment:

python
b = "Hello, World!"

This line assigns the string "Hello, World!" to the variable b.

1. String Slicing:

python
print(b[-5:-2])

This line prints a slice of the string b. Here's how the slicing works:

• Negative Indexing: In Python, negative indices count from the end of the string. So, -1 is the last character, -2 is the second last, and so on.
• Slice b[-5:-2]: This means "start from the 5th character from the end and go up to (but not including) the 2nd character from the end."

Let's visualize the string with indices:

H e l l o , W o r l d !
0 1 2 3 4 5 6 7 8 9 10 11 12
-13-12-11-10 -9 -8 -7 -6 -5 -4 -3 -2 -1

So, b[-5:-2] corresponds to the characters orl from the string "Hello, World!".

Therefore, the output of print(b[-5:-2]) will be:

orl

Python Strings

Get more here

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