没有定义的字母变量在Python中有效

Question

My code is this:

word = input('enter a word:')
for letter in word:
        print( letter)

Output:

enter a word:tree
t
r
e
e

Is letter an in-built variable?

Answer 1

You don't need to declare variables in Python. The variable is defined in the for-loop directly. Python has only very few keywords . It is also not a built-in constant .

I recommend going through a Python tutorial . You might also want to try exercism .

Variable Annotations

The following stuff is only relevant for Python 3.6+. No matter which Python version you use, you can ignore it. If you are a very early beginner, you probably should ignore it.

You can use variable annotations to "declare" a variable. PEP 526 introduces them. They look like this:

foo: str
int: bar

Answer 2

Python is a language which doesn't require to declare variables beforehand, like Pascal or any versions of C. In the moment you use a new variable, it is considered as being declared. In your case, letter is declared in the for loop.

Answer 3

"Variables declaration" in Python is implicit, in fact we better use name binding . There are many ways of name binding in Python, for is one of them.

For more details, you can have a look at binding-of-names .

There is one very important implicit fact: Python will pre-compute all names before executing code of one certain scope.

def test_1():
    b = a + 1

def test_2():
    b = a + 1
    for a in range(3):
        print(a)

# NameError: name 'a' is not defined
test_1()

# UnboundLocalError: local variable 'a' referenced before assignment
test_2()

In test_1 , when executing b = a + 1 , the error is name 'a' is not defined .

In test_2 , when executing b = a + 1 , the error is local variable 'a' referenced before assignment . That is to say, in test_2 , when executing b = a + 1 , Python already knew that a is a local variable, it just has not been bound to an object, so the error is UnboundLocalError .