Why doesn't Python print return values?

问题:

I'm taking a class in Algorithms and Data Structures (Python). In the book there is an example of a "stack" with methods that return certain values. In the book, these values are printed when the methods are called. However, when I run the program nothing is printed. I have to print the return value myself. Is this a difference between Python 2 and 3, or am I doing something wrong? Here is the code.

class Stack:
    def __init__(self):
        self.items = []

    def isEmpty(self):
        return self.items == []

    def push(self, item):
        self.items.append(item)

    def pop(self):
        return self.items.pop()

    def peek(self):
        return self.items[len(self.items)-1]

    def size(self):
        return len(self.items)

s=Stack(); s.push(5)
s.size()
s.isEmpty()
s.peek()

So, this should print "1", "False" and "5". But it doesn't.

回答1:

At the interactive interpreter, Python will print the repr of expression values (except None) as a convenience. In a script, you have to print manually, as automatic printing would be highly awkward to work around in a script.

回答2:

Why should it print if you haven't included a print statement? Try:

print s.size()
print s.isEmpty()
print s.peek()

回答3:

I assume that you have the code in your input file, say a.py. The values of s.size() etc. are ignored in such a case. On the other hand, if you type in anything like that in interactive python session, the values will be printed automatically for you. The best way to try all those calls is: remove all operations on s from your input file, leaving only Stack definition there. Then use

python -i a.py

This will load your file with Stack definition, and then interactive mode (so called REPL) will be available, where you can try s=Stack(); s.push(5) etc.

回答4:

You can either

s=Stack()
s.push(5)
print s.size()
print s.isEmpty()
print s.peek()

or replace your return statements with print.