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Difference between mutation, rebinding, copying value, and assignment operator

Asked 2012-01-31T03:29:26.363
11
#!/usr/bin/env python3.2

def f1(a, l=[]):
    l.append(a)
    return(l)

print(f1(1))
print(f1(1))
print(f1(1))

def f2(a, b=1):
    b = b + 1
    return(a+b)

print(f2(1))
print(f2(1))
print(f2(1))

In f1 the argument l has a default value assignment, and it is only evaluated once, so the three print output 1, 2, and 3. Why f2 doesn't do the similar?

Conclusion:

To make what I learned easier to navigate for future readers of this thread, I summarize as the following:

  • I found this nice tutorial on the topic.

  • I made some simple example programs to compare the difference between mutation, rebinding, copying value, and assignment operator.

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2 Answers

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This is a slightly tricky case. It makes sense when you have a good understanding of how Python treats names and objects. You should strive to develop this understanding as soon as possible if you're learning Python, because it is central to absolutely everything you do in Python.

Names in Python are things like a, f1, b. They exist only within certain scopes (i.e. you can't use b outside the function that uses it). At runtime a name refers to a value, but can at any time be rebound to a new value with assignment statements like:

a = 5
b = a
a = 7

Values are created at some point in your program, and can be referred to by names, but also by slots in lists or other data structures. In the above the name a is bound to the value 5, and later rebound to the value 7. This has no effect on the value 5, which is always the value 5 no matter how many names are currently bound to it.

The assignment to b on the other hand, makes binds the name b to the value referred to by a at that point in time. Rebinding the name a afterwards has no effect on the value 5, and so has no effect on the name b which is also bound to the value 5.

Assignment always works this way in Python. It never has any effect on values. (Except that some objects contain "names"; rebinding those names obviously effects the object containing the name, but it doesn't affect the values the name referred to before or after the change)

Whenever you see a name on the left side of an assignment statement, you're (re)binding the name. Whenever you see a name in any other context, you're retrieving the (current) value referred to by that name.

With that out of the way, we can see what's goi

answered 2012-01-31T06:06:32.233
0

In f1 you are storing the value in an array or better yet in Python a list where as in f2 your operating on the values passed. Thats my interpretation on it. I may be wrong

answered 2012-01-31T03:54:09.563

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