Roadmap¶
- 異常
- 異常處理
- while、break、continue
- 補充知識-引發異常
異常 exception¶
當發生異常的時候，程式會中斷無法繼續執行
In [1]:
print(123
In [2]:
print(SyntaxError)
In [3]:
print("Hi")
print(123)
In [4]:
for i in range(3):
print(i)
In [5]:
for i in range(3):
print(i)
print("i",i)
In [6]:
print(name)
In [7]:
print(3/0)
In [8]:
print(int("Hi"))
In [9]:
print("Hi"+123)
In [10]:
l = []
print(l[0])
In [11]:
d = {}
print(d["key"])
目前我們學的語法，可以怎麼避免掉這些異常呢？
d = {}
print(d["key"])
print(d["other_key"])
In [12]:
d = {}
print(d.get("key","nothing"))
print(d.get("other_key","nothing"))
In [13]:
if "key" in d:
print(d["key"])
else:
print("nothing")
if "other_key" in d:
print(d["other_key"])
else:
print("nothing")
用函式的寫法
In [14]:
def get(d,key):
if key in d:
return d[key]
else:
return "nothing"
d = {}
print(get(d,"key"))
print(get(d,"other_key"))
把print改放在函式裡
In [15]:
def my_print(d,key):
if key in d:
print(d[key])
else:
print("nothing")
d = {}
my_print(d,"key")
my_print(d,"other_key")
還有其他的資料型別同時存在...
d = {}
l = []
print(d["key"])
print(l[0])
print(d["other_key"])
In [16]:
def my_print(data,ik):
if isinstance(data,list):
if ik in range(0,len(data)):
print(data[ik])
else:
print("nothing")
if isinstance(data,dict):
if ik in data:
print(data[ik])
else:
print("nothing")
d = {}
l = []
my_print(d,"key")
my_print(l,0)
my_print(d,"other_key")
異常處理¶
try:
可能會發生異常的程式碼
except:
發生任何異常要執行的程式碼
except 異常型別名:
發生異常型別要執行的程式碼
except 異常型別名 as 名稱:
發生異常型別要執行的程式碼，且為此異常物件取名稱
except (異常型別名1,異常型別名2,....):
發生異常型別1,2,....要執行的程式碼
except (異常型別名1,異常型別名2,....) as 名稱:
發生異常型別1,2,....要執行的程式碼，且為此異常物件取名稱
else:
未發生異常時要執行的程式碼
finally:
不論如何都會執行的程式碼
注意：¶
except,finally至少要有一存在except有多種寫法else可有可無- 直接用
except雖然很方便，但因為沒有綁定名稱，會讓你找不到bug所在
d = {}
l = []
print(d["key"])
print(l[0])
print(d["other_key"])
In [17]:
def my_print(data,ik):
try:
print(data[ik])
except:
print("nothing")
d = {}
l = []
a = 1234
my_print(d,"key")
my_print(l,0)
my_print(d,"other_key")
my_print(a,1)
In [18]:
def my_print(data,ik):
try:
print(data[ik])
except (KeyError,IndexError):
print("nothing")
except:
print("資料型別錯誤")
d = {}
l = []
a = 1234
my_print(d,"key")
my_print(l,0)
my_print(d,"other_key")
my_print(a,1)
user_input = input("請輸入欲計算的除法內容")
try:
a,b = user_input.split("/")
print("a={}, b={}".format(a,b))
a = eval(a) # 可直接把字串當成運算式執行
b = int(b)
print("a={}, b={}".format(a,b))
print(a/b)
except ZeroDivisionError:
print("分母為0，無法計算")
except ValueError as e:
print("計算錯誤：{}".format(e))
except:
print("計算錯誤")
else:
print("計算成功")
finally:
print("程式結束")
以下輸入會有各種結果
5+3/2=> 8/2 = 4.05/0=> ZeroDivisionError5/Hi=> ValueError5=> ValueError5+3)/2=> SyntaxError
在異常狀況不做任何處理 pass¶
pass就只是為了符合Python句法結構，實際上什麼事都不會做。
In [19]:
try:
output = 5/0
except:
pass
finally:
print("結束")
異常名稱的範圍¶
except 指派的名稱只會存在自己的範圍裡
In [20]:
try:
output = 5/0
except ZeroDivisionError as e:
print(e)
finally:
print(e)
In [21]:
error = None
try:
output = 5/0
except ZeroDivisionError as e:
error = e
print(e)
finally:
print(error)
finally 和 return¶
一般的函式都是以最先的執行的retun為回傳值，
但當使用了異常處理，try與finally都有return時，會以finally的return為回傳值。
In [22]:
def f():
return "try"
return "finally"
return "f"
print(f())
In [23]:
def f():
try:
return "try"
finally:
return "finally"
return "f"
print(f())
實務上會遇到的狀況-以網路爬蟲為例¶
- 可能網址不存在 => 404 Not Found
- 可能整個網站掛了 => ERR_CONNECTION_TIMED_OUT
In [24]:
import requests
from lxml import etree
resp = requests.get("http://www.slime.com.tw/tutorial/os.htm")
html = resp.text
page = etree.HTML(html)
url_set = set(page.xpath("//a[contains(@href,'http')]/@href"))
for url in url_set:
if "www.slime.com.tw" not in url:
print(url)
resp = requests.get(url, timeout=0.5)
resp.raise_for_status() # 如果有發生404等錯誤請求，引發異常
html = resp.text
In [25]:
import requests
from lxml import etree
resp = requests.get("http://www.slime.com.tw/tutorial/os.htm")
html = resp.text
page = etree.HTML(html)
url_set = set(page.xpath("//a[contains(@href,'http')]/@href"))
for url in url_set:
if "www.slime.com.tw" not in url:
print(url)
try:
resp = requests.get(url, timeout=2)
resp.raise_for_status()
except Exception as e:
print(e)
else:
html = resp.text
練習-檔案異常處理¶
之前在做檔案處理時，
如果輸出的檔案是在某個資料夾底下，但該資料夾卻不存在，會引發異常，
請用try/except處理：若資料夾不存在，印出「資料夾不存在」。
- 建議使用
except 異常型別名:的寫法- 可以先看看資料夾不存在是產生怎樣的錯誤
可能會有這兩種寫法，試試看try/except應該放在哪裡
當然直接包最外層也是可以(超級懶惰xD)，但不是最好的寫法=D
Type 1
fo = open("tmp/test.txt","w")
fo.write("data")
fo.close()
Type 2
with open("tmp/test.txt","w") as fo:
fo.write("444")
while、break、continue¶
while¶
if 條件成立(True):
要做的事
要做的事
while 條件成立(True):
重複要做的事
重複要做的事
for 名稱 in 可迭代的資料型別:
重複要做的事
重複要做的事
for- 處理「已知」次數的重複狀況
- 常用在取出元素
while- 處理「未知」次數的重複狀況
In [26]:
# 發5張牌(撲克牌每一張都是完全不同的牌面)
import random
number_list = ["A","J","Q","K"]
for i in range(1,10+1):
number_list.append(str(i))
color_list = ["黑桃","梅花","紅心","方塊"]
my_card = []
while(len(my_card)<5):
card = random.choice(color_list)+random.choice(number_list)
if card not in my_card:
my_card.append(card)
print (card)
while "I am satisfied with my care" not in word:
make_happy()
while hp>0:
fighting()
print("You are dead")
today_count = 0
while is_in_opentime(now_time):
if is_someone_comin:
today_count += 1
print(today_count)
In [27]:
import random
with open("choice.txt") as fi:
data = fi.read()
food_list = data.split("\n")
print(random.choice(food_list))
import random
food_list = ["牛肉麵","麥當勞","7-11"]
def get_suggestion():
return random.choice(food_list)
while True:
user_input = input()
if "謝謝" in user_input:
print("祝你有個愉快的一餐")
elif ("好餓喔" in user_input) or ("換一個" in user_input):
print(get_suggestion())
else:
print("我聽不懂你在說什麼")
print("程式結束")
break¶
跳出最近的迴圈(for、while)
import random
food_list = ["牛肉麵","麥當勞","7-11"]
def get_suggestion():
return random.choice(food_list)
while True:
user_input = input()
if "謝謝" in user_input:
print("祝你有個愉快的一餐")
break ###
elif ("好餓喔" in user_input) or ("換一個" in user_input):
print(get_suggestion())
else:
print("我聽不懂你在說什麼")
print("程式結束")
In [28]:
# dresscode要全部的人都穿藍色衣服才能進場
people = ["blue","black","blue","blue","blue"]
for color in people:
print("now color =",color)
if color !="blue":
break
In [29]:
# dresscode要全部的人都穿藍色衣服才能進場(雙重迴圈)
people = [["blue","blue"],["black","blue","blue"],["blue"]]
for group in people:
print("now_group =",group)
for color in group:
print(" now color =",color)
if color !="blue":
break
continue¶
繼續最近的迴圈(for、while)
In [30]:
# 單價<10的物品不進貨
item_list = [("orange",20),("lemon",5),("banana",15),("apple",25)]
stock = []
for item in item_list:
if item[1]<10:
continue
stock.append(item)
print(stock)
In [31]:
# 程式當然不只一種寫法
item_list = [("orange",20),("lemon",5),("banana",15),("apple",25)]
stock = []
for item in item_list:
if item[1]>10:
stock.append(item)
print(stock)
Hint:¶
# 程式生成一個1~100的隨機數
import random
program_number = random.randrange(1, 100)
print("偷看答案",program_number)
# 讓使用者能輸入猜的數字，提示字為「請輸入你想要猜的數字
user_input = input("請輸入你想要猜的數字")
# 輸入的字串轉成數字
user_number = int(user_input)
# 判斷對錯
if program_number == user_number:
print("猜對了")
else:
if program_number>user_number:
print("比這個數字更大")
else:
print("比這個數字更小")
補充知識-引發異常¶
In [32]:
a = 5
if a==5:
print("fine")
else:
raise
In [33]:
a = 10
if a==5:
print("fine")
else:
raise # 當前無任何異常 => RuntimeError
In [34]:
a = 10
if a==5:
print("fine")
else:
raise ValueError
再次引發異常¶
In [35]:
try:
output = 5/0
except ZeroDivisionError as e:
print("分母不得為0")
In [36]:
try:
output = 5/0
except ZeroDivisionError as e:
print("分母不得為0")
raise # 當前異常為ZeroDivisionError
建立新的錯誤訊息-保留原本訊息¶
raise 異常型別名/異常物件(訊息) from 名稱
In [37]:
try:
output = 5/0
except ZeroDivisionError as e:
s = "分母不得為0"
raise ZeroDivisionError(s) from e
建立新的錯誤訊息-捨棄原本訊息¶
raise 異常型別名/異常物件(訊息) from None
In [38]:
try:
output = 5/0
except ZeroDivisionError as e:
s = "分母不得為0"
raise ZeroDivisionError(s) from None
In [39]:
assert 5+5==10,"First"
assert 5<10,"Second"
print('Done. Time to Check.')
In [40]:
assert 5+5==10,"First"
assert 5>10,"Second"
print('Done. Time to Check.')
學習資源對應¶
- Python 程式設計入門 (適用於 2.x 與 3.x 版) 葉難
- 12 異常
- ec. 12.5 引發異常：自行定義異常類別
- 13 延伸學習
- 13.5 動態執行：eval
- 12 異常
書中講過的內容¶
- 物件、型別轉換：int(字串)、str(數字)
- 字串：split,strip,replace,isalpha,lower
- 串列：append
- 序列共同介面：index,count,len,切片
- 字典、集合
- 判斷式、in
- 迴圈
- 檔案
- 函式,排序
- 好用模組
- 異常
書中沒講到的內容¶
- 06 函式：遞迴、高階函式、產生器 generator
- 08 其他容器型別
- 09 再談函式：再談遞迴、裝飾器 decorator、函數式程式設計
- 10 物件導向程式設計 OOP：類別class、繼承、多重繼承、後設類別
- 13 延伸學習：程式撰寫風格和實務作法、命令列參數、圖形介面、並行處理
Python 各種領域的程式庫/框架¶
- [檔案處理與路徑]：os、shutil、sys
- [網路爬蟲]：urllib、requests、lxml、beautiful_soup、scrapy、selenium
- [資料庫串接]：sqlite3(sqlite)、MySQLdb(MySQL)、pymssql(MSSQL)、Psycopg(PostgreSQL)
- [自然語言]：NLTK、jieba
- [統計應用]：pandas、numpy、scipy、matplotlib
- [機器學習]：scikit-learn、TensorFlow
- [影像處理]：PIL、opencv
- [網站架設]：Django、Flask
- [網路分析]：scapy、socket
- [GUI設計]：tkinter、PyQt
- [軟硬整合]：raspberry pi 樹莓派、Arduino、puSerial
- [遊戲開發]：pygame
- [App開發]：kivy
- [各種服務的API串接]：Bot