If you want 'while false' functionality, you need not. Try while not fn: instead.

while True means loop forever. The while statement takes an expression and executes the loop body while the expression evaluates to (boolean) "true". True always evaluates to boolean "true" and thus executes the loop body indefinitely. It's an idiom that you'll just get used to eventually! Most languages you're likely to encounter have equivalent idioms.

Note that most languages usually have some mechanism for breaking out of the loop early. In the case of Python it's the break statement in the cmd == 'e' case of the sample in your question.

There is no do-while loop in Python.

This is a similar construct, taken from the link above.

 while True:
     do_something()
     if condition():
        break

When you say the code doesn't work, I assume you mean the code inside the while loop doesn't execute in the 2nd example.

While loops in python have format

while BOOLEAN:
    CODE

where BOOLEAN is an expression which evaluates to either True or False. If the BOOLEAN evaluates to True, then CODE is executed. If not, CODE is skipped over.

In your second example, BOOLEAN is the expression

not valid

valid has value True so,

not valid

is not True which evalutes to False. Therefore the CODE doesn't execute.

Use break to exit a loop:

while True:
    ser = serial.Serial("/dev/ttyUSB0", 4800, timeout =1)
    checking = ser.readline();
    if checking.find(",,,,"):
        print "not locked yet"
    else:
        print "locked and loaded"
        break

The True and False line didn't do anything in your code; they are just referencing the built-in boolean values without assigning them anywhere.

If there's nothing you want to do in between checking the conditions, then the cleanest solution is like this:

if cond1 and cond2 and cond3:
    do_thing()

Assuming you do want to do some things in between checking the conditions, here are a few options, in order of subjective Pythonicity.

The cleanest solution is probably to put your code into a function and use return instead of break; note that you can write this as a local function, there is no need to expose every function declared in your code. However, if the function is useful enough then you might consider exposing it anyway.

def do_thing_on_conditions():
    if not cond1: return
    if not cond2: return
    if not cond3: return
    do_thing()
do_thing_on_conditions()

The do/while(false) idiom is essentially a loop structure which iterates once and only once, so you can write it as a for loop over a sequence of length 1:

for _ in range(1):
    if not cond1: break
    if not cond2: break
    if not cond3: break
    do_thing()

Finally, you could do this: but I don't recommend it, because (like the do/while(false) idiom) you have to read all the way to the end to know this will only iterate once.

while True:
    if not cond1: break
    if not cond2: break
    if not cond3: break
    do_thing()
    break