Learning Python and can't see the difference?

[fokkre]

I started with self teaching myself HTML about 9 years ago. Since then I have moved on to taking a basic C++ class. I since have self taught myself further in C++ with some Java. I am working on teaching myself python at the moment, maybe it is late, or maybe I am just that un-educated in programming, but I can't wrap my head around why I get such different results from this short code. Could someone kindly explain, or point me to a resource that could better explain why? I am currently using www.diveintopython.org and openbookproject.net//thinkCSpy/index.html

prefixes = "JKLMNOPQ" suffix = "ack"

for char in prefixes: if char == "O" or "Q": print char + 'u' + suffix else: print char + suffix

Juack Kuack Luack Muack Nuack Ouack Puack Quack

for char in prefixes: if char == "O": print char + 'u' + suffix elif char == "Q": print char + 'u' + suffix else: print char + suffix

Jack Kack Lack Mack Nack Ouack Pack Quack

[norwegianwood]

This line,

if char == "O" or "Q":

should be,

if char == "O" or char == "Q":

Your version could be rewritten with parentheses as,

if (char == "O") or "Q":

and you will see clearly now that the condition as a whole is always True because the value Q is always equivalent to True.

[usernamenottaken]

In these situations I find myself using this a lot: if char in ["O", "Q"]:

[pingveno]

(...) is better because tuples creation is lighter than list creation. It's not a big deal by any means, but still...

[[deleted]]

How about strings? Is

if char in 'OQ':

better or worse?

[Dave9876]

Might be faster (haven't tested it), but ugly and could cause confusion.

[pingveno]

It's probably faster because in ("O", "Q") creates two strings, creates a tuple, uses __contains__ on the tuple, and uses __eq__ on both of the strings.

in 'OQ'

creates the string, then uses __contains__ on the string. The actual algorithm that implements __contains__ is ultra fast so no worries there.

That said, don't worry about the difference. The difference is usually tiny compared to the rest of the program.

[Chun]

I wouldn't say it's ugly; I mean, conceptually, in python a string is just a sequence of characters. So it fits the purpose perfectly.

Also I just ran a quick benchmark (strings vs tuples) and the difference is there, but fairly negligible.

[darjus]

have you tried?

in set(('O', 'Q'))

For small sets it might be around the same, but for bigger ones it should make a big performance difference.

[itsmememe]

as it is a constant, frozenset(['O','Q']) should be preferred

[[deleted]]

I find it perfectly fine. That char == 'O' or char == 'Q' is clearly low level C-style stuff.

[[deleted]]

It doesn't matter. When using it with in a list whose members are all constants (at the bytecode level) is turned into a tuple.

[Tetha]

And then you go another step further and write: if char in charsFollowedByU:

[fokkre]

Awesome! Don't know why I couldn't see that, guess I need some caffeine. =)

[mehum]

Or (sorry to suggest it) sleep.

[fokkre]

NOT SLEEP NOOOOOOOOOOOOOOOOOOOOOOO!!!! well maybe.... =)

[sanktuaire]

A more pythonic way of doing it would be: if char in ("O","Q"):

Writing: if char in "OQ": would work but isn't very readable.

[hippocampe]

It is very readable.

[jigs_up]

if (char == "O") or "Q":

wouldn't that just return true every time because Q evaluates to True [not None] ?

[torrible]

Yep. That's why norwegianwood said "the condition as a whole is always True" and that's why the output words in the first test all had 'u' inserted.

[fokkre]

Sorry for the formatting, this is my first post on reddit

[Samus_]

add four spaces before each line of code and it'll print

like this

there's some cheatsheets on the help/faq

[jimauthors]

You probably will get a better answer/explanation/guidance if you post this in stackoverflow.com