Python中的__str__方法

Question

I can't manage to get this __str__ to work. I created a class,

class Hangman : 

and then

def __str__(self) : 
return "WORD: " + self.theWord + "; you have " + \
self.numberOfLives + "lives left"

there's an init statement and assignment in the program but I can't get the thing to work! the only way I can do it is by doing this, but surely what's the point of using __str__

def __str__(self) :
    print("WORD: {0}; you have {1} lives left".\
    format(self.theWord,self.numberOfLives))

Code:

theWord = input('Enter a word ')
numberOfLives = input('Enter a number ')
hangman = Hangman(theWord,numberOfLives)
Hangman.__str__(hangman)

Output:

>>> 
Enter a word Word
Enter a number 16
>>> 

using the print method, output:

>>> 
Enter a word word
Enter a number 16
WORD: word; you have 16 lives left
>>> 

Answer 1

Hangman.__str__(hangman)

This line will just call the __str__ method. So does this btw. which is the preferred way to do it (in general, don't call special methods directly):

str(hangman)

str and the __str__ method are just there to convert the object into a string, but not to print it. For example you could just as well log it to a file, so printing wouldn't always be appropriate.

Instead, if you want to print it, just print it:

print(hangman)

print will automatically call str() on the object and as such use the type's __str__ method to convert it to a string.

Answer 2

Like this post says: https://mail.python.org/pipermail/tutor/2004-September/031726.html

 >>> class A:
...   pass
...
 >>> a=A()
 >>> print a
<__main__.A instance at 0x007CF9E0>

If the class defines a __str__ method, Python will call it when you call 
str() or print:
 >>> class B:
...   def __str__(self):
...     return "I'm a B!"
...
 >>> b=B()
 >>> print b
I'm a B!

Quoting

To recap: when you tell Python to "print b", Python calls str(b) to get the string representation of b. If the class of b has a __str__ method, str(b) becomes a call to b.__str__(). This returns the string to print.

Answer 3

Hangman.__str__(hangman) isn't a command to print the string representation of hangman , it's an expression that evaluates to the string representation of hangman .

If you type that manually at the interactive prompt, you'll get the value of the expression printed, because the interactive prompt does that as a convenience. Having that line in a script (or in a function you call) will not print anything - you need to actually tell python to print it, with print(hangman) .

Answer 4

This code works:

class Hangman(object):

    def __init__(self, theWord, numberOfLives):
        self.theWord = theWord
        self.numberOfLives = numberOfLives

    def __str__(self) :
        return "WORD: " + self.theWord + "; you have " + \
               self.numberOfLives + " lives left"

if __name__ == '__main__':
    theWord = input('Enter a word ')
    numberOfLives = input('Enter a number ')
    hangman = Hangman(theWord,numberOfLives)
    print(hangman)

Output:

>>> 
Enter a word word
Enter a number 16
WORD: word; you have 16 lives left