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One of the first things programmers do when software starts misbehaving is to stick in `print()` calls. This is great if you don't mind extra output, but there's a better way to get diagnostic info back from your app!
The Python logging library docs
New terms:
- level - The level that you want to start logging messages at. Any message at this level or above will be logged.
- Logging Levels:
- CRITICAL
- ERROR
- WARNING
- INFO
- DEBUG
- NOTSET
Example:
import logging
try:
[1, 2, 3].remove(4)
except ValueError:
logging.error("tried to remove an invalid value")
print("Sorry, that value doesn't exist.")
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[MUSIC]
0:00
We've gotten pretty good at printing
things out to the screen when we
0:04
make scripts and games.
0:07
Quite often though you don't want to print
everything out to the user,
0:09
you want to collect information about how
something is running, or
0:12
create a report when something blows up.
0:15
This is where logging comes in.
0:18
Let's go back to workspaces and see how to
use this really handy tool.
0:20
So this dd_game.py is a slightly more
advanced version of
0:23
the game that we built in Python
collections.
0:28
The one where your in a maze, trying to
find the door.
0:30
It draws the map,
0:33
like before, but it shows some breadcrumbs
for where the player has been.
0:34
So we'll scroll down here, and you can see
here we're,
0:39
we're drawing like a dot if they've been
there, 'kay.
0:42
So I'd like to know where the player,
monster and door start out but
0:46
obviously I don't wanna tell the player
where the door and the monster are then
0:52
they can just get away easily, right, so
this is a great time to use logging.
0:57
To do this we have to import the logging
library.
1:02
I'll do this to the top and since I'm
actually really picky
1:06
I'm gonna put it before random because l
comes before r,
1:11
alphabetical yeah, it's, it's kind of
ridiculous.
1:17
So I can actually use it, so let's go
ahead and use it, and
1:22
we'll just do logging.info You won't see
this, and
1:26
then we'll do logging.warn OH NO.
1:33
All right, so we're gonna save those and
1:35
let's go down here, and run our script and
oh, look.
1:40
There's our message, it says, OH NO.
1:47
So that came out,
1:49
but the one that said, You won't see this
didn't, didn't come out.
1:50
We didn't get the info message let's go
and quit.
1:53
So what can we do?
1:56
First of all our, our info one didn't show
up we want these things to
2:01
show up if we log them, and the second one
is that it printed directly to the screen,
2:06
which means it'd be shown to the player
and we don't want to show it to
2:11
the player, we want to print it to, maybe
a file cuz we gotta keep our secrets.
2:15
We gotta keep our secrets, secret, right?
2:19
So to do that, we're gonna use this basic
config method
2:22
This is one place where Python's foolish
consistency kinda comes through.
2:26
Some really old packages use camel
2:30
case names You kinda just have to remember
it, or, or look it up each time.
2:35
So let's do logging.basicConfig, and
2:40
we're gonna give it a filename, and we're
gonna say game.log,
2:45
and we're gonna give it a level and it's
gonna be logging.DEBUG.
2:52
Okay, so the file name argument as
2:59
you can probably guess tells logging what
file to log into.
3:02
By default it does standard out which is
why things get printed.
3:08
We want it to log into a file called
game.log.
3:11
The path, the file name is relative, so
game.log is gonna be in the same directory
3:14
as our script, if you wanted to go to say
/temp or something like that,
3:19
then you'd put that in as well.
3:24
So filename makes sense, but what's with
level?
3:27
What does level mean?
3:30
Well, level tells the logger what level to
start paying attention to,
3:31
for messages, it'll ignore any messages
with a lower level.
3:37
Well, what levels are available?
3:42
Well, there are six log levels and
3:46
their names make it pretty obvious what
they're for.
3:47
They're critical, error, warning, info,
debug, and notset.
3:49
You won't use notset very often, I'd
almost say never.
3:57
Debug and info are for information about
the running of an application, so
4:01
it's your debug information.
4:05
This just massive stream of information to
keep track of what everything is doing.
4:07
Info is like oh, this is a good thing to
monitor.
4:12
Warning is for
4:16
keeping track of questionable things that
happen during your app, like you know,
4:17
maybe an exception occurs, and you want to
remember that it happened, and
4:22
then error and critical are for when
things go horribly wrong.
4:27
We want our logger to catch any and all
messages that have a level set.
4:32
So we're setting the level to debug.
4:36
Okay, so let's blog something.
4:39
Let's go down here, to line 73, or so,
where's that at?
4:42
[BLANK_AUDIO]
4:48
It's down here.
4:51
Yeah, 69 okay, close enough.
4:53
We've got these three variables, monster,
door and player location.
4:56
I wanna record what these things are, so
5:00
we're gonna call logging.info, which if
you haven't noticed calling.info, .debug,
5:03
.warning, whatever, that logs a message at
that particular level.
5:09
So we're gonna say monster door and
5:14
player, and then we're gonna call .format
on that, and
5:23
we're gonna pass in monster, door and
player location, 'kay.
5:28
So, that should log the starting position.
5:37
Let's test it out.
5:40
Right, cuz that's what we do, we test
things out.
5:43
So python dd.game.
5:46
All right, nothing got printed out, which
is exactly what we wanted.
5:47
We don't wanna give the players an unfair
advantage, this time.
5:51
So let's go ahead and quit actually, and
if we do an ls here,
5:56
we see there's this file game.log.
6:02
Now sometimes, things wont show up over
here right away, so
6:06
let's just go ahead, and we'll just do cat
game.log and that'll print out game.log,
6:16
and we see that we have an info message
logged from the root logger.
6:22
Monster's at 0,1, door's at 0, 0, player's
at 1, 2.
6:27
So this can be really tricky getting to
that door instead of hitting that monster.
6:30
So, there's our message, that's great!
6:36
I'm sure as you look through this
dd_game.py you'll find other places to
6:39
add in more logging.
6:43
You can log the current number of moves
that the players made,
6:45
or if you made a version where the monster
moves,
6:48
maybe log where the monster is after each
player move.
6:50
As you can see, logging is a really useful
tool to use with your scripts.
6:54
You can record an information passed in by
the user, and then ask for
6:58
those logs if something doesn't work like
you expected it to.
7:01
You can also log exceptions in your except
blocks.
7:04
There's really nothing more frustrating
than trying to debug a script when you
7:07
can't see what's going on.
7:10
Actually that brings me to our next and
final topic the Python debugger.
7:12
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