# Introduction to Python
Table of Content
[TOC]
## 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
```python=
>>> name = input('What is your name?\n')
vayne # 使用者輸入
>>> print ('Hi, %s.' % name)
Hi, vayne.
```
## Variable
### Types
```python=
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
```python=
>>> x = 5 # int
>>> y = 1.9 # float
>>> print(x + y)
6.9
>>> x = 10 # 修改變數值
>>> print(x + y)
11.9
```
```python=
>>> 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)
```python=
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)
```python=
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) ]
```python=
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}
```python=
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
```python=
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
```python=
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:
```python=
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
# apple
# banana
# cherry
```
Loop through the letters in the word “banana”:
```python=
for x in "banana":
print(x)
# b
# a
# n
# a
# n
# a
```
### The `range()` Function
```python=
for x in range(6):
print(x)
# 0
# 1
# 2
# 3
# 4
# 5
```
Print i as long as i is less than 6:
```python=
i = 1
while i < 6:
print(i)
i += 1
# 1
# 2
# 3
# 4
# 5
```
### The `break` Statement
```python=
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:
```python=
i = 0
while i < 6:
i += 1
if i == 3:
continue
print(i)
# 1
# 2
# 4
# 5
# 6
```
## Function
### Creating a Function
```python=
def my_function():
print("Hello from a function")
```
### Calling a Function
```python=
def my_function():
print("Hello from a function")
my_function()
```
### Arguments
```python=
def my_function(name):
print("Hi " + name)
my_function("David") # Hi David
```
### Number of Arguments
This function expects 2 arguments, and gets 2 arguments:
```python=
def my_function(fname, lname):
print(fname + " " + lname)
my_function("David", "Jiang") # David Jiang
```
This function expects 2 arguments, but gets only 1:
```python=
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:
```python=
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.
```python=
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:
```python=
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:
```python=
class MyClass:
x = 5
```
### Create Object
Now we can use the class named MyClass to create objects:
```python=
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.
```python=
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
```python=
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。
```python=
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。
```python=
from math import floor, ceil
from numpy import zeros as zs
zs((5, 1))
floor(1.5)
ceil(1.3)
```
###### tags: `python` `guide`
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