Python輸入和異常與C++

Question

我想用pythonic的方式儘可能接近地複製下面的C++代碼，並且輸入和異常處理儘可能地接近。我取得了成功，但可能不是我想要的。我希望退出類似於C++輸入隨機字符的程序，在這種情況下它是'q'。 while條件中的cin對象與python創建while時的方式不同。另外我想知道將2個輸入轉換爲int的簡單行是否是一種適當的方式。最後，在Python代碼中，「再見！」從未運行，因爲強制應用程序關閉的EOF（控制+ z）方法。有怪癖，總體而言，我對python中所需的代碼越少感到滿意。

額外：如果您查看最後一次打印語句中的代碼，是將var和字符串一起打印的好方法嗎？

任何簡單的技巧/技巧都是受歡迎的。

C++

#include <iostream> 

using namespace std; 

double hmean(double a, double b); //the harmonic mean of 2 numbers is defined as the invese of the average of the inverses. 

int main() 
{ 
    double x, y, z; 
    cout << "Enter two numbers: "; 

    while (cin >> x >> y) 
    { 
     try  //start of try block 
     { 
      z = hmean(x, y); 
     }   //end of try block 
     catch (const char * s)  //start of exception handler; char * s means that this handler matches a thrown exception that is a string 
     { 
      cout << s << endl; 
      cout << "Enter a new pair of numbers: "; 
      continue;  //skips the next statements in this while loop and asks for input again; jumps back to beginning again 
     }          //end of handler 
     cout << "Harmonic mean of " << x << " and " << y 
      << " is " << z << endl; 
     cout << "Enter next set of numbers <q to quit>: "; 
    } 
    cout << "Bye!\n"; 

    system("PAUSE"); 
    return 0; 
} 

double hmean(double a, double b) 
{ 
    if (a == -b) 
     throw "bad hmean() arguments: a = -b not allowed"; 
    return 2.0 * a * b/(a + b); 
} 

的Python

class MyError(Exception): #custom exception class 
    pass 

def hmean(a, b): 
    if (a == -b): 
     raise MyError("bad hmean() arguments: a = -b not allowed") #raise similar to throw in C++? 
    return 2 * a * b/(a + b); 

print "Enter two numbers: " 

while True: 
    try: 
     x, y = raw_input('> ').split() #enter a space between the 2 numbers; this is what .split() allows. 
     x, y = int(x), int(y) #convert string to int 
     z = hmean(x, y) 
    except MyError as error: 
     print error 
     print "Enter a new pair of numbers: " 
     continue 

    print "Harmonic mean of", x, 'and', y, 'is', z, #is this the most pythonic way using commas? 
    print "Enter next set of numbers <control + z to quit>: " #force EOF 

#print "Bye!" #not getting this far because of EOF 

Answer 1

對於功能hmean我會試圖執行return語句，並引發異常，如果a等於-b：

def hmean(a, b): 
    try: 
     return 2 * a * b/(a + b) 
    except ZeroDivisionError: 
     raise MyError, "bad hmean() arguments: a = -b not allowed" 

要在字符串變量插值的方法format是一種常見的替代：

print "Harmonic mean of {} and {} is {}".format(x, y, z) 

最後，你可能想，如果鑄造X或Y時int一個ValueError被提升到使用except塊。

Answer 2

這是我想要拋棄的一段代碼。類似的東西是不是在C++容易實現，但它通過分離關注使事情在Python更清晰：

# so-called "generator" function 
def read_two_numbers(): 
    """parse lines of user input into pairs of two numbers""" 
    try: 
     l = raw_input() 
     x, y = l.split() 
     yield float(x), float(y) 
    except Exception: 
     pass 

for x, y in read_two_numbers(): 
    print('input = {}, {}'.format(x, y)) 
print('done.') 

它採用所謂的發電機的功能，只有處理輸入解析到輸入從計算分開。這並不是「儘可能接近」，而是你所要求的「pythonic方式」，但我希望你會發現這個有用。另外，我冒昧地使用浮點數來代替數字。

還有一件事：升級到Python 3，版本2不再開發，只是接收錯誤修正。如果你不依賴任何僅適用於Python 2的庫，你應該不會感覺太大。