Python if vs try-except

Question

I was wondering why the try-except is slower than the if in the program below.

def tryway():     try:         while True:             alist.pop()     except IndexError:         pass  def ifway():     while True:         if alist == []:              break         else:             alist.pop() if __name__=='__main__':     from timeit import Timer     alist = range(1000)     print "Testing Try"     tr = Timer("tryway()","from __main__ import tryway")     print tr.timeit()     print "Testing If"     ir = Timer("ifway()","from __main__ import ifway")     print ir.timeit() 

The results I get are interesting.

Testing Try 2.91111302376 Testing If 0.30621099472 

Can anyone shed some light why the try is so much slower?

Answer 1

You're setting alist only once. The first call to "tryway" clears it, then every successive call does nothing.

def tryway():     alist = range(1000)     try:         while True:             alist.pop()     except IndexError:         pass  def ifway():     alist = range(1000)     while True:         if alist == []:             break         else:             alist.pop() if __name__=='__main__':     from timeit import Timer     print "Testing Try"     tr = Timer("tryway()","from __main__ import tryway")     print tr.timeit(10000)     print "Testing If"     ir = Timer("ifway()","from __main__ import ifway")     print ir.timeit(10000)  >>> Testing Try >>> 2.09539294243 >>> Testing If >>> 2.84440898895 

Answer 2

Exception handling is generally slow in most languages. Most compilers, interpreters and VMs (that support exception handling) treat exceptions (the language idiom) as exceptions (uncommon). Performance optimization involves trade-offs and making exceptions fast would typically mean other areas of the language would suffer (either in performance or simplicity of design).

At a more technical level, exceptions generally mean that the VM/interpretter (or the runtime execution library) has to save a bunch of state and begin pulling off all the state on the function call stack (called unwinding) up until the point where a valid catch (except) is found.

Or looking at it from a different viewpoint, the program stops running when an exception occurs and a "debugger" takes over. This debugger searches back through the stack (calling function data) for a catch that matches the exception. If it finds one, it cleans things up and returns control to the program at that point. If it doesn't find one then it returns control to the user (perhaps in the form of an interactive debugger or python REPL).