python try statement or if statement

try should be used when something would return an error and would otherwise cause your program to stop/crash. An example of this would be:

try:
    import pandas
except:
    print("Unable to import pandas.  Module not installed")

In this example your program will attempt to import the pandas module. If this fails it will then print out a line of text and continue running.

if statements are used to decided when to do something or not based on the returned logic. The key difference is that logic IS returned and not an error.

if x > 10:
   print("This is a large number")
else: 
   print("This is a small number")

With this example, if 'x' did not exist it would produce an error, no more code will be executed, and the program will crash. The main difference between IF and TRY is whether logic is returned as true/false or is something just plains fails.

With your specific example it is important to know if the captcha appearing or not will break your code. Does the logic boil down to captcha = false or does captcha not exist at all and logic fails entirely?

Q: How do you define sometimes captcha doesn't appear (1%, 20%, 50%, ...)?

A: Maybe 5% of the time captcha doesn't appear.

In this case, I prefer to use Exception handling: do stuff and if something goes wrong, fix it

try:
  # > 95% of the time the code below works when the captcha appears
except SomeException:
  # < 5% of the time the code is called when the captcha doesn't appear

IMHO, you have not really 2 different codes: you have one and a fallback solution, it's really different than:

if x % 2:
    ...
else:
    ...