Introduction to Python

Table of Content

Introduction to Python

Introduction

Python is free and easy to learn
超多 standard libary, community
支援領域廣(machine learning, data science, web servers, DB…)
語法簡潔、易寫易讀

Python 語法特徵

直譯式語言，開發、除錯速度快
使用縮排(indention)分隔程式段落
多種基本的資料型別: numbers, strings, lists and dictionaries
支援 OOP 用法(classes and multiple inheritances)
進階用法: generators, list comprehensions

Input/Output





>>> name = input('What is your name?\n')
vayne  # 使用者輸入

>>> print ('Hi, %s.' % name)
Hi, vayne.

Variable

Types









a = 123                   # int
b = 1.23                  # float
c = 1.2 + 2.3j            # complex
d = (True or False)       # boolean
e = [1, 2, 3]             # list
f = (1, 2, 3)             # tuple
g = {'a': 1, 'b': 2}      # dictionary
h = 'abc'                 # string
l = None                  # None

Number








>>> x = 5                 # int
>>> y = 1.9               # float
>>> print(x + y)
6.9

>>> x = 10                # 修改變數值
>>> print(x + y)
11.9







>>> 3 / 2                 # true division: 1.5
>>> 3 // 2                # division: 1
>>> 3 % 2                 # mod: 1
>>> 3 ** 2                # power: 9
>>> float(3)              # 3.0 (float)
>>> int(1.5)              # 1(int)
>>> round(1.5)            # 2 (int)

List (mutable)




heros = ['Zed', 'Ezreal']        # heros: ['Zed', 'Ezreal'] 
heros[1] = 'Vayne'               # heros: ['Zed', 'Vayne'] 
heros.append('Yasuo')            # heros: ['Zed', 'Vayne', 'Yasuo']
heros.extend(['jinx, Twitch'])   # heros: ['Zed', 'Vayne', 'Yasuo', 'Jinx', 'Twitch']

Tuple (immutable)






heros = ('Zed', 'Ezreal')        # heros: ('Zed', 'Ezreal') 
heros = 'Zed', 'Ezreal'          # heros: ('Zed', 'Ezreal')
heros[1] = 'Vayne'               # error

mid, adc = heros                 # mid: 'Zed'
                                 # adc: 'Ezreal'

Indexing and Slicing

Usage: [ start: end (不含) : step (default 1) ]






a = [0, 1, 2, 3, 4]
len(a)                           # 5
a[0] + a[-1]                     # 0 + 4 = 4
a[2:-1]                          # [2, 3]
a[0:5:2]                         # [0, 2, 4]
a[::-1]                          # [4, 3, 2, 1, 0]

Dict (mutable)

Usage: {key: value}





d = dict({'a': 1, 'b': [1, 2]})  # d: {'a': 1, 'b': [1, 2]}
d['a'] = '3'                     # d: {'a': 3, 'b': [1, 2]}
d.items()                        # [('a', 3), ('b', [1, 2])]       
d.keys()                         # ['a', 'b']
d.values()                       # [3, [1, 2]]

Str (immutable)

Usage: 單引號、雙引號、三引號










a = '單引號'
b = "雙引號"
c = '''
    三引號
    多行
    '''
d = len('a b c')       # 5
e = 'a b c'.split()    # ['a', 'b', 'c']
f = a + ' ' + b        # '單引號 雙引號'
a[0] = 'e'             # error

Control Flow

If, elif, else

Equals: a == b
Not Equals: a != b
Less than: a < b
Less than or equal to: a <= b
Greater than: a > b
Greater than or equal to: a >= b











a = 200
b = 33

if b > a:
    print("b is greater than a")
  
elif a == b:
    print("a and b are equal")
  
else:
    print("a is greater than b")

For, While

Print each fruit in a fruit list:







fruits = ["apple", "banana", "cherry"]
for x in fruits:
    print(x)
# apple
# banana
# cherry

Loop through the letters in the word “banana”:








for x in "banana":
    print(x)
# b
# a
# n
# a
# n
# a

The `range()` Function








for x in range(6):
    print(x) 
# 0 
# 1 
# 2 
# 3 
# 4 
# 5

Print i as long as i is less than 6:









i = 1
while i < 6:
    print(i)
    i += 1
# 1
# 2
# 3 
# 4 
# 5

The `break` Statement









i = 1
while i < 6:
  print(i)
  if i == 3:
    break
  i += 1
# 1
# 2
# 3

The `continue` Statement

Continue to the next iteration if i is 3:











i = 0
while i < 6:
  i += 1
  if i == 3:
    continue
  print(i)
# 1
# 2
# 4 
# 5 
# 6

Function

Creating a Function


def my_function():
    print("Hello from a function")

Calling a Function




def my_function():
    print("Hello from a function")

my_function()

Arguments




def my_function(name):
    print("Hi " + name)

my_function("David")  # Hi David

Number of Arguments

This function expects 2 arguments, and gets 2 arguments:




def my_function(fname, lname):
  print(fname + " " + lname)

my_function("David", "Jiang")    # David Jiang

This function expects 2 arguments, but gets only 1:




def my_function(fname, lname):
  print(fname + " " + lname)

my_function("Weieie")            # error

Arbitrary Arguments, *args

If the number of arguments is unknown, add a * before the parameter name:




def my_function(*kids):
  print("The youngest child is " + kids[2])

my_function("Emil", "Tobias", "Linus")        # The youngest child is Linus

Keyword Arguments

You can also send arguments with the key = value syntax.

This way the order of the arguments does not matter.




def my_function(child3, child2, child1):
  print("The youngest child is " + child3)

my_function(child1 = "Emil", child2 = "Tobias", child3 = "Linus")

Arbitrary Keyword Arguments, **kwargs

If the number of keyword arguments is unknown, add a double ** before the parameter name:




def my_function(**kid):
  print("His last name is " + kid["lname"])

my_function(fname = "Tobias", lname = "Refsnes")

Classes

Create a Class

Create a class named MyClass, with a property named x:


class MyClass:
  x = 5

Create Object

Now we can use the class named MyClass to create objects:


p1 = MyClass()
print(p1.x)        # 5

The `init()` Function

The __init __() function is called automatically every time the class is being used to create a new object.









class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

p1 = Person("John", 36)

print(p1.name)          # John
print(p1.age)           # 36

class的概念是屬性集合，而不是所有物

Object Methods










class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

  def myfunc(self):
    print("Hello my name is " + self.name)

p1 = Person("John", 36)
p1.myfunc()

Modules

`import … as …`

使⽤該 package 的函式、物件要前綴 package 的名稱。





import math import numpy as np 
 
np.zeros((5, 1)) 
math.floor(1.5) 
math.ceil(1.3)

`from … import … as …`

將該 package 底下的函式、物件加⼊當前 namespace 中。






from math import floor, ceil 
from numpy import zeros as zs 
 
zs((5, 1)) 
floor(1.5) 
ceil(1.3)