Why do I not have to define the variable in a for loop using range(), but I do have to in a while loop in Python?

Question

I have the following code using a for loop:

    total = 0
    for num in range(101):
       total = total + num
       print(total)

Now the

Answer 1

In python there is no need, in most cases, to define/declare variables.

The rule is that if you write (assign) a variable then the variable is a local variable of the function; if you only read it instead then it's a global.

Variables assigned at top-level (outside any function) are global... so for example:

x = 12     # this is an assignment, and because we're outside functions x
           # is deduced to be a global

def foo():
    print(x)     # we only "read" x, thus we're talking of the global

def bar():
    x = 3        # this is an assignment inside a function, so x is local
    print(x)     # will print 3, not touching the global

def baz():
    x += 3       # this will generate an error: we're writing so it's a
                 # local, but no value has been ever assigned to it so it
                 # has "no value" and we cannot "increment" it

def baz2():
    global x     # this is a declaration, even if we write in the code
                 # x refers to the global
    x += 3       # Now fine... will increment the global

The for statement is simply a loop that writes to a variable: if no declaration is present then the variable will be assumed to be a local; if there is a global or nonlocal declaration then the variable used will have the corresponding scope (nonlocal is used to write to local variable of the enclosing function from code in a nested function: it's not used very frequently in Python).