Suppose I have two functions one after another in the same python file:
def A(n):
B(n-1)
# if I add A(1) here, it gives me an error
def B(n):
if n <= 0:
return
else:
A(n-1)
When the interpreter is reading A, B is not yet defined, however this code does not give me an error. This confuses me, because I thought that Python programs are interpreted line by line. How come the attempt to call B within A doesn't give an error immediately, before anything is called?
My understanding is that, when def is interpreted, Python adds an entry to some local name space locals() with {"function name": function address}, but as for the function body, it only does a syntax check:
def A():
# this will give an error as it isn't a valid expression
syntax error
def B():
# even though x is not defined, this does not give an error
print(x)
# same as above, NameError is only detected during runtime
A()
Do I have it right?
A SyntaxError will be caught at compile time, but most other errors (NameError, ValueError, etc.) will be caught only at runtime, and then only if that function is called.
"if I have written a function, if its not called in my test.." - and that is why you should test everything.
Some IDEs will raise warnings in various situations, but the best option is still to conduct thorough testing yourself. This way, you can also check for errors that arise through factors like user input, which an IDE's automated checks won't cover.