Is there a way to do conditionals inside Python (3) for loops?

You can use range() if you have the step as some constant increment (like i++,i+=10,etc). The syntax is -

range(start,stop,step)

range(start, stop, step) is used as a replacement for for (int i = start; i < stop; i += step). It doesn't work with multiplication, but you can still use it (with break) if you have something like i < stop && condition.

The equivalent loop for the one you mentioned in question can be =>

for(int i=0;i<20;i*=2)  // C/C++ loop

# Python - 1
i = 0
while i < 20 :    # Equivalent python loop
    # Code here
    i*=2

If you are willing to use flag as well as a condition, you will have to do it as =>

// C/C++
bool flag = true; 
for(int i=0;i<20&&flag;i*=2)  // C/C++ loop

# Python - 1
i,flag = 1,True
while not flag and i < 20 :    # Equivalent python loop
    # Code here
    i*=2 

Hope this helps !

Not directly. A for loop iterates over a pre-generated sequence, rather than generating the sequence itself. The naive translation would probably look something like

flag = True
i = 1
while i < 20:
    if not flag:
        break
    ...
    if some_condition:
        flag = False
    i *= 2

However, your code probably could execute the break statement wherever you set flag to False, so you could probably get rid of the flag altogether.

i = 1
while i < 20:
    ...
    if some_condition:
        break
    i *= 2

Finally, you can define your own generator to iterate over

def powers_of_two():
    i = 1
    while True:
        yield i
        i *= 2


for i in powers_of_two():
    ...
    if some_condition:
        break
    

The for loops in Python are not like loops in C. They are like the for-each loops applied to iterables that came out in Java 7:

for (String name: TreeSet<String>(nameList) ) {
   //Your code here
}

If you want control over your iterator variable, then a while or for loop with a break in it is probably the cleanest way to achieve that kind of control.

This might be a good time to look into finding time to do a tutorial on Python comprehensions. Even though they are not directly applicable to your question, that is the feature that I appreciate most having come from Java about five years ago.