【Python學習記錄——從入門到放棄】九、檔案與異常
阿新 • • 發佈：2018-11-21
本文使用的書籍是《Python程式設計：從入門到實踐》
本文使用的是Python3.6
一、從檔案中讀取資料
這一節主要講的是如何讀取檔案，無非就是幾個方法而已。
- 讀取整個檔案
首先建立一個檔案
3.1415926335
8979323846
2643383279
儲存在pi_digits.txt
接下來我們用程式開啟並讀取這個檔案
with open('pi_digits.txt') as file_object:
contents = file_object.read()
print(contents)
注意，程式和pi_digits.txt要放在同一目錄下。
根據程式輸出的結果，發現輸出末尾多了一個空行，我們要使用rstrip()來刪除多出來的空行。
with open('pi_digits.txt') as file_object:
contents = file_object.read()
print(contents.rstrip())
-
檔案路徑
當你需要讀取放在資料夾的檔案的時候，就要把路徑放在open裡
windows記得是用
Linux和OS X使用/
with open(‘路徑\檔名’) as file_object -
逐行讀取
with open('pi_digits.txt') as file_object:
for line in file_object:
print - 建立一個包含檔案各行內容的列表
filename = 'pi_digits.txt'
with open(filename) as file_object:
lines = file_object.readlines()
for line in lines:
print(line.rstrip())
- 使用檔案的內容
filename = 'pi_30_digits.txt'
with open(filename) as file_object:
lines = file_object.readlines( 這樣，我們就得到了檔案中的資料了。
- 包含一百萬位的大型檔案
filename = 'pi_million_digits.txt'
with open(filename) as file_object:
lines = file_object.readlines()
pi_string = ''
for line in lines:
pi_string += line.strip()
print(pi_string[:52] + "...")
print(len(pi_string))
這個跟上面的操作差不多，沒有什麼不同，就是所用資料更大而已。
- 圓周率值中包含你的生日嗎
filename = 'pi_million_digits.txt'
with open(filename) as file_object:
lines = file_object.readlines()
pi_string = ''
for line in lines:
pi_string += line.strip()
birthday = input("Enter your birthday, in the form mmddyy: ")
if birthday in pi_string:
print("Your birthday appears in the first million digit of pi!")
else:
print("Your birthday does not appear in the first million digits of pi.")")
動手試一試：
# 10-1
filename = 'learning_python.txt'
with open(filename) as file_object:
print('第一種：')
print(file_object.read().rstrip())
with open(filename) as file_object:
print('第二種：')
for line in file_object:
print(line.rstrip())
with open(filename) as file_object:
lines = file_object.readlines()
print("第三種：")
for line in lines:
print(line.rstrip())
# learning_python.txt
In python you can print.
In python you can input.
In python you can make a list.
# 10-2
with open(filename) as file_object:
lines = file_object.readlines()
for line in lines:
print(line.replace('python', 'Java').rstrip())
二、寫入檔案
這一節主要講的是寫檔案，跟讀檔案差不多233。
- 寫入空檔案
filename = 'programming.txt'
with open(filename, 'w') as file_object:
file_object.write('I love programming.')
注意：現在open()裡除了檔名，還有另一個引數’w’，這個引數代表的是寫入模式。
除此之外，還有其他模式：
| 引數 | 模式 |
|---|---|
| w | 寫入模式 |
| r | 讀取模式 |
| a | 附加模式 |
| r+ | 讀取和寫入模式 |
【重點注意！！！注意！！注意了！】
如果你使用’w’寫入模式的時候，在開啟檔案時千萬要注意，如果資料夾沒有檔案的話，程式會自動建立檔案；如果有檔案的話，程式會刪除原來就有的檔案，在建立一個檔案，也就是覆蓋原檔案。
- 多行寫入
filename = 'programming.txt'
with open(filename, 'w') as file_object:
file_object.write('I love programming.\n')
file_object.write('I love creating new game.\n')
記得每一句話在需要換行的地方都要加’\n’，不然語句會擠在一起。
- 附加到檔案
filename = 'programming.txt'
with open(filename, 'a') as file_object:
file_object.write('I also love finding meaning in large datasets.\n')
file_object.write('I love creating apps that can run in a brower.\n')
這裡使用’a’來表示進行附加模式，將語句寫到檔案的後面，而不是覆蓋檔案。
動手試一試：
# 10-3
filename = 'guest.txt'
while True:
name = input('請輸入你的名字：')
if name == 'quit':
break
with open(filename, 'a') as file_object:
file_object.write(name)
# 10-4
import time
filename = 'guest_book.txt'
while True:
name = input('請輸入你的名字：')
if name == 'quit':
break;
print(name + ', 歡迎你登入！')
with open(filename, 'a') as file_object:
file_object.write(name + '於' + str(time.strftime("%Y-%m-%d %H:%M:%S", time.localtime())) + '進行登入' + '.\n')
三、異常
python使用異常的特殊物件來管理程式執行期間發生的錯誤。
如果你未對異常進行處理，程式將停止，並顯示一個traceback，其中包含有關異常的報告。
- 處理ZeroDivisionError異常
下面來看一種導致Python引發異常的簡單錯誤。
print(5/0)
OK，這個程式執行時肯定會報錯，眾所周知，數字是不能除以0的，233。
下面我們來看一下如何處理。
- 使用try-except程式碼塊
try:
pritn(5/0)
except:
print("You can't divide by zero!")
這裡我們可以看到try-except程式碼塊的使用方法，在try中寫入可能或必定233會出錯的程式碼，當try中程式碼出錯時，程式將會執行except中的語句。
- 使用異常避免崩潰
發生錯誤是，如果程式還有工作沒有完成，妥善地處理錯誤就尤其重要。
當你寫完一個程式的時候，總有一些人為因素導致的錯誤，就比如上面的ZeroDivisionError，雖然你知道不能除零，但總有人會試試233。
print("Give me two numbers, and I'll divide them.")
print("Enter 'q' to quit")
while True:
first_number = input('\nFirst number: ')
if first_number == 'q':
break;
second_number = input('Second number: ')
if second_number == 'q':
break;
answer = int(first_number)/int(second_number)
print(answer)
類似這樣，當然你以後可能會寫更復雜的程式，更應該要寫好異常，有助於發現錯誤。
- else程式碼塊
print("Give me two numbers, and I'll divide them.")
print("Enter 'q' to quit")
while True:
first_number = input('\nFirst number: ')
if first_number == 'q':
break;
second_number = input('Second number: ')
if second_number == 'q':
break;
try:
answer = int(first_number)/int(second_number)
except ZeroDivisionError:
print('You can't divide by 0!')
else:
print(answer)
在上面程式碼中，如果沒有發生錯誤，程式執行完try程式碼塊後，會執行else程式碼塊，不執行except程式碼塊；如果發生錯誤，首先在try程式碼塊中報錯的語句是不能執行的，然後程式會執行except語句，不執行else程式碼塊。
- 處理FileNotFoundError異常
filename = 'alice.txt'
try:
with open(filename) as f_obj:
contents = f_obj.read()
except FileNotFoundError:
msg = "Sorry, the file " + filename + " does noe exist."
print(msg)
- 分析文字
filename = 'alice.txt'
try:
with open(filename) as f_obj:
contents = f_obj.read()
except FileNotFoundError:
msg = "Sorry, the file " + filename + " does noe exist."
print(msg)
else:
# 計算檔案大致包含多少個單詞
words = contents.split()
num_words = len(words)
print("The file " + filename + " has about " + str(num_words) + " words.")
- 使用多個檔案
def count_words(filename):
try:
with open(filename) as f_obj:
contents = f_obj.read()
except FileNotFoundError:
msg = "Sorry, the file " + filename + " does noe exist."
print(msg)
else:
# 計算檔案大致包含多少個單詞
words = contents.split()
num_words = len(words)
print("The file " + filename + " has about " + str(num_words) + " words.")
filename = 'alice.txt'
count_words(filename)
- 失敗時一聲不吭
def count_words(filename):
try:
with open(filename) as f_obj:
contents = f_obj.read()
except FileNotFoundError:
pass
else:
# 計算檔案大致包含多少個單詞
words = contents.split()
num_words = len(words)
print("The file " + filename + " has about " + str(num_words) + " words.")
filename = 'alice.txt'
count_words(filename)
- 決定報告那些錯誤
……
動手試一試：
# 10-6
while True:
first_number = input('\nFirst number: ')
if first_number == 'quit':
break;
second_number = input('Second number: ')
if second_number == 'quit':
break;
try:
result = int(first_number) + int(second_number)
except ValueError:
print('請輸入數字！')
else:
print(result)
# 10-7
while True:
first_number = input('\nFirst number: ')
if first_number == 'quit':
break;
second_number = input('Second number: ')
if second_number == 'quit':
break;
try:
result = int(first_number) + int(second_number)
except ValueError:
print('請輸入數字！')
else:
print(result)
# 10-8
def read_file(filename):
try:
with open(filename) as f_obj:
lines = f_obj.readlines();
except FileNotFoundError:
print('File not found')
else:
for line in lines:
print(line.rstrip())
cats = 'cats.txt'
dogs = 'dogs.txt'
read_file(cats)
rad_file(dogs)
# 10-9
def read_file(filename):
try:
with open(filename) as f_obj:
lines = f_obj.readlines();
except FileNotFoundError:
pass
else:
for line in lines:
print(line.rstrip())
cats = 'cats.txt'
dogs = 'dogs.txt'
read_file(cats)
rad_file(dogs)
# 10-10
def count_words(filename):
try:
with open(filename) as f_obj:
contents = f_obj.read()
except FileNotFoundError:
pass
else:
print(contents.lower().count('the'))
filenames = ['Slavery in History.txt', 'John Lackland.txt']
for filename in filenames:
count_words(filename)
四、儲存資料
這一節中使用學習使用json進行儲存資料。
- 使用json.dump()和json.load()
import json
numbers = [2, 3, 5, 7, 11, 13]
filename = 'numbers.json'
with open(filename, 'w') as f_obj:
json.dump(numbers, f_obj)
使用json.dump()儲存資料
import json
filename = 'numbers.json'
with open(filename) as f_obj:
numbers = json.load(f_obj)
print(numbers)
使用json.load()讀取資料
- 儲存和讀取使用者生成的資料
import json
filename = 'username.json'
try:
with open(filename) as f_obj:
username = json.load(f_obj)
except FileNotFoundError:
username = input("What is your name?")
with open(filename, 'w') as f_obj:
json.dump(username, f_obj)
print("we'll remember you when you come back, " + username + "!")
else:
print("Welcome back, " + username + "!")
這段程式碼是前面的整和，對各塊知識的應用。
- 重構
這一小節裡要學習使方法的任務簡單化……
首先，先把上面的程式碼變成一個函式：
import json
def greet_user():
filename = 'username.json'
try:
with open(filename) as f_obj:
username = json.load(f_obj)
except FileNotFoundError:
username = input("What is your name?")
with open(filename, 'w') as f_obj:
json.dump(username, f_obj)
print("we'll remember you when you come back, " + username + "!")
else:
print("Welcome back, " + username + "!")
greet_user()
接下來，我們把讀取使用者名稱的方法分離，重構greet_user()方法
import json
def get_stored_username():
"""如果儲存了使用者名稱，就獲取它"""
filename = 'username.json'
try:
with open(filename) as f_obj:
username = json.load(f_obj)
except FileNotFoundError:
return None
else:
return username
def greet_user():
"""問候使用者，並指出名字"""
username = get_stored_username()
if username:
print("Welcome back, " + username + "!")
else:
username = input("What is your name?")
filename = 'username.json'
with open(filename, 'w') as f_obj:
json.dump(username, f_obj)
print("we'll remember you when you come back, " + username + "!")
greet_user()
最後，再把寫入資料拆出來，重構greet_user()
import json
def get_stored_username():
"""如果儲存了使用者名稱，就獲取它"""
filename = 'username.json'
try:
with open(filename) as f_obj:
username = json.load(f_obj)
except FileNotFoundError:
return None
else:
return username
def get_new_username():
"""提示使用者輸入使用者名稱"""
username = input("What is your name?")
filename = 'username.json'
with open(filename, 'w') as f_obj:
json.dump(username, f_obj)
return username
def greet_user():
"""問候使用者，並指出名字"""
username = get_stored_username()
if username:
print("Welcome back, " + username + "!")
else:
username = get_new_username()
print("we'll remember you when you come back, " + username + "!")
greet_user()
動手試一試：
# 10-11
# 寫入檔案
import json
filename = 'favorite_number.json'
favorite_number = int(input('input your favorite number: '))
with open(filename, 'w') as f_obj:
json.dump(favorite_number, f_obj)
# 讀取檔案
import json
filename = 'favorite_number.json'
with open(filename) as f_obj:
favorite_number = json.load(f_obj)
print("I know your favorite number! It's " + str(favorite_number))
# 10-12
import json
filename = 'favorite_number.json'
try:
with open(filename) as f_obj:
favorite_number = json.load(f_obj)
except:
favorite_number = int(input('input your favorite number: '))
with open(filename, 'w') as f_obj