# 常用方法

## elif

等於其他語言中的「else if」

## 類似三元運算子

```python
flag = True if x.isClick() else False
```

{% code title="等同" %}

```python
if x.isClick():
  flag = True
else:
  flag = False
```

{% endcode %}

## \~ (波浪符號)

「反、NOT」之意。

{% code title="可處理「減去」資料" %}

```python
data = [ 'a', 'b' ]
data_dup = [ 'b' ]
data[~data_dup] # =[ 'a' ]
```

{% endcode %}

## either

### Union

Union\[X, Y] means either X or Y.

{% code title="EX" %}

```python
Union[str, None] #不是空值就是String
```

{% endcode %}

### Optional

```python
Optional[X] #等同 Union[X, None]
Optional[str]="AAA" #不填入時會是預設值"AAA"
```

## None

是空值等同null

### is None

判斷物件是否為「空」

```python
if o is None:
if o is not None:
```

{% hint style="warning" %}
類型為NoneType，但用isinstance會無法判斷，「空」的東西沒有實際上的類型，有點類似JS的undefined
{% endhint %}

## 可換行文字

```python
"""XXX""" #等同JS中`XXX`
```

{% hint style="info" %}
不指定給變數時可當「多行註解」使用
{% endhint %}

## len = length

```python
len(object)
```

## Type

```python
type(x) #x的類型
```

## isinstance

判斷物件類型，搭配type(o)使用

isinstance(o, type)

{% code title="EX" %}

```python
isinstance(row[i_row], datetime)
```

{% endcode %}

## 算數

```python
x // y	#X除以Y，只取整數解
x % y	#求X除以Y的餘數
x ** y	#X的Y次方
```

## lambda

用於宣告一個簡易函式，基本只有一行的處理，想多行需提出宣告為def。

方便使用於函數變數需要是簡易函式時。

{% code title="語法結構" %}

```python
lambda [參數]: [函式本體]
```

{% endcode %}

{% code title="EX" %}

```python
>>> sorted(('Justin', 'openhome', 'momor'), key = lambda name: name[-1]) 
['openhome', 'Justin', 'momor']
>>> max = lambda n1, n2: n1 if n1 > n2 else n2
>>> max(10, 5)
10
```

{% endcode %}

## 切片(索引值的使用)

```
seq[start:end:step]
```

seq為任意序列(list、tuple，也可以是string)

start = 取值起始位置，預設0。\
end = 取值結束位置(不包含)，預設seq長度。\
step = 切片資料列的步長(取值間格)，預設1。

```python
a = [1, 2, 3, 4, 5]

print(a[-1]) #倒數第一個
>>> 5

print(a[::-1]) #倒敘
>>> [5, 4, 3, 2, 1]
```

> 書.page69(圖示很清楚)

## linspace（分N等份）

```python
import numpy as np
print(np.linspace(1, 10, 10))
> [ 1.  2.  3.  4.  5.  6.  7.  8.  9. 10.]

print(np.linspace(0.1, 1.0, 10))
> [0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1. ]
```

> 參考：<https://vimsky.com/zh-tw/examples/usage/numpy-linspace-python.html>

## 程式碼太長時...

如果程式碼太長寫成一行不便於閱讀，可以使用\對程式碼進行折行。

```python
is_leap = year % 4 == 0 and year % 100 != 0 or \
          year % 400 == 0
```


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