Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 4: Journal app and file I/O
Lecture: Complex conditionals and, or, and not

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0:00 Let's come back to our event loop for a moment.
0:02 So when I run this app, I can list the items,
0:05 and I can an add an entry and hit exit,
0:08 but if I hit like enter, you can see I get kind of this weird
0:11 like hey we don't understand nothing,
0:14 maybe I'd like an empty entry that just mean
0:16 hey let's just exit, save and exit,
0:18 so we can change our code really quickly here,
0:21 first of all, let me change this to be something
0:23 other than a non-existent empty pointer
0:28 we want to come up here and we want to have
0:30 a complex conditional that says
0:32 when the command is not the x and it's not empty
0:36 so in many languages, you use symbols like the &&
0:42 or double pipe to say and and or,
0:44 Python we just say and or or,
0:48 ok, so if we want to say the command is not equal to x,
0:52 and the command has some value in it,
0:56 I could say like it's not equal to none,
0:59 or the length is greater than 0, or something like this,
1:02 but in Python remember the truthiness of collections and strings are collections
1:08 are if they are not empty then they are true,
1:10 and if they are empty then they are false.
1:12 Maybe that's a bad choice of words of being empty like this.
1:15 Ok, so this will almost work if we run this,
1:19 and when I say list then enter,
1:21 it says we don't understand that and then boom, it exits.
1:24 So we need to add another bit down here,
1:26 say if the command is not X,
1:30 and there is some sort of command entered here, right,
1:34 which is not empty, then save this error.
1:37 If we wanted to say it's not X and there's nothing in here
1:39 we would say and not command.
1:41 Right, so there is ways to combine this with parenthesis,
1:45 and all the boolean logic you can do in regular programming.
1:49 All right, so here let's list the entries,
1:50 and hit enter boom, goodbye.