1
00:00:00,700 --> 00:00:02,270
Whenever we run a Python script,

2
00:00:02,270 --> 00:00:05,010
by default it runs in the directory
where the script is saved.

3
00:00:05,010 --> 00:00:06,313
That's why imports work when we try

4
00:00:06,313 --> 00:00:08,099
to import another module
in the same directory.

5
00:00:08,099 --> 00:00:10,808
I know there've been been languages and
systems in the past though,

6
00:00:10,808 --> 00:00:13,750
where our script would've been executed
in the directory where the Python

7
00:00:13,750 --> 00:00:14,580
executable lived.

8
00:00:14,580 --> 00:00:16,518
That would be really annoying.
We would constantly have to change

9
00:00:16,518 --> 00:00:19,260
the directory that our
script was working in.

10
00:00:19,260 --> 00:00:24,040
Almost every bit of code that we use in
this course will be using the OS library.

11
00:00:24,040 --> 00:00:24,763
It's build into Python.

12
00:00:24,763 --> 00:00:29,429
If you find yourself unable to run a file
or follow along with a bit of code,

13
00:00:29,429 --> 00:00:33,209
make sure you've imported
the library by using import os.

14
00:00:33,209 --> 00:00:38,898
Now we can see what directory we're in,
also know as the current working directory

15
00:00:38,898 --> 00:00:44,035
by using the getcwd function,
os.getcwd or current working directory.

16
00:00:45,577 --> 00:00:48,437
And I can see that I'm here,
in my users folder,

17
00:00:48,437 --> 00:00:52,587
inside of a projects directory,
inside of a file_systems directory.

18
00:00:52,587 --> 00:00:55,307
This will of course work the same,
no matter what directory we're in.

19
00:00:55,307 --> 00:00:58,897
For example,
let's make a directory here named backups.

20
00:00:58,897 --> 00:01:05,940
So I'm gonna open up a tab, and I'm gonna
make a directory here named backups.

21
00:01:05,940 --> 00:01:08,370
And I'm gonna change my
directory into backups, and

22
00:01:08,370 --> 00:01:09,940
then I'm gonna use Python there.

23
00:01:11,062 --> 00:01:14,884
And I'm gonna import os, and
I'm gonna do os.getcwd and

24
00:01:14,884 --> 00:01:19,060
I see now that I'm inside
of the backups directory.

25
00:01:19,060 --> 00:01:20,550
So now we can see where we are.

26
00:01:20,550 --> 00:01:22,720
What if we find out that we're
somewhere we don't be though?

27
00:01:22,720 --> 00:01:25,100
Or we're not yet
where we want to do our work?

28
00:01:25,100 --> 00:01:28,680
We can use the chdir, or
change directory function,

29
00:01:28,680 --> 00:01:31,350
to change the directory that
Python's currently working in.

30
00:01:31,350 --> 00:01:33,710
So I can do os.chdir('..').

31
00:01:33,710 --> 00:01:38,830
And then,
os.get current working directory.

32
00:01:38,830 --> 00:01:43,304
And now I see that I'm in the file_systems
directory and not the backups directory.

33
00:01:43,304 --> 00:01:46,310
But wait, what's this dot dot thing?

34
00:01:46,310 --> 00:01:49,500
Let's take a second to talk about
relative and absolute paths.

35
00:01:49,500 --> 00:01:53,260
We have two types of paths on computers,
absolute and relative.

36
00:01:53,260 --> 00:01:56,700
An absolute path is the entire full
path all the way from the root

37
00:01:56,700 --> 00:01:59,310
to whatever location you want.

38
00:01:59,310 --> 00:02:05,250
If my next course was in
C:\Users\Kenneth\my_next_course.py,

39
00:02:05,250 --> 00:02:07,330
this is an absolute path.

40
00:02:07,330 --> 00:02:11,080
A relative path though is a path to
one location from another location.

41
00:02:11,080 --> 00:02:15,920
If my current working directory is
/home/workspace/backups/ I could point to

42
00:02:15,920 --> 00:02:21,380
the directory above /home/workspace
as the /../ directory.

43
00:02:21,380 --> 00:02:23,970
Double dots means move up one directory.

44
00:02:23,970 --> 00:02:26,450
A single dot means the current directory.

45
00:02:26,450 --> 00:02:28,580
You can use as many of
these as you want too, so

46
00:02:28,580 --> 00:02:32,230
you can chain dot dots to
move up several directories.

47
00:02:32,230 --> 00:02:35,550
Relative paths can also move down
the tree and won't use single or

48
00:02:35,550 --> 00:02:37,000
double dots at all.

49
00:02:37,000 --> 00:02:41,040
Usually these start with just the next
directories name and no leading slash.

50
00:02:41,040 --> 00:02:43,580
If I was in
the /home/workspaces directory,

51
00:02:43,580 --> 00:02:46,915
I could point to the backups
directory by just saying, backups/.

52
00:02:48,230 --> 00:02:52,150
So a relative path says how to get there
from here, while an absolute path covers

53
00:02:52,150 --> 00:02:55,000
getting to there from the absolute
beginning of your file system.

54
00:02:55,000 --> 00:02:58,510
Python has a handy function to tell
us if a path is relative or absolute.

55
00:02:58,510 --> 00:03:00,331
It lives in the path part of OS.

56
00:03:00,331 --> 00:03:05,080
So we can do os.path.isabs and
we can pass in a path.

57
00:03:08,190 --> 00:03:10,120
And we get back that that is true.

58
00:03:10,120 --> 00:03:13,070
That is a absolute path.

59
00:03:13,070 --> 00:03:20,010
If we were to give a relative path though,
like /workspaces, we would get back false.

60
00:03:20,010 --> 00:03:21,930
Because that's not an absolute path.

61
00:03:21,930 --> 00:03:25,359
Now, if you're on Windows and I can't show
you this sadly because I'm on a Mac at

62
00:03:25,359 --> 00:03:28,300
this point, you would have to do
your path a little bit differently.

63
00:03:28,300 --> 00:03:34,180
You'd have to do something
like this to do the C:\\.

64
00:03:34,180 --> 00:03:36,440
Now I have to use the double
backslashes there for

65
00:03:36,440 --> 00:03:39,540
Windows style paths, and
it's because the single backslash

66
00:03:39,540 --> 00:03:42,320
is usually used to show that
the next character is special.

67
00:03:42,320 --> 00:03:43,670
It's called an escape.

68
00:03:43,670 --> 00:03:46,340
If we don't use the double backslash,
Python thinks we're trying to use some

69
00:03:46,340 --> 00:03:50,850
special character here,
where the slash capital U is.

70
00:03:50,850 --> 00:03:53,090
And that's actually not
a special character.

71
00:03:54,490 --> 00:03:58,340
And notice, too,
here that I can make up paths.

72
00:03:58,340 --> 00:04:02,238
Python's just checking to see if it looks
like an absolute path, not whether or

73
00:04:02,238 --> 00:04:04,054
not it actually is an absolute path.

74
00:04:04,054 --> 00:04:08,005
If os.path.isabs gives us back false,
we know that it's a relative path.

75
00:04:08,005 --> 00:04:10,660
If it gives us back true,
we know it's an absolute path.

76
00:04:10,660 --> 00:04:12,649
There's a lot more in the os.path.

77
00:04:12,649 --> 00:04:14,520
We're gonna spend a while in
there during this course.

78
00:04:16,483 --> 00:04:19,530
If you haven't already, be sure to
check out the docs for the OS module.

79
00:04:19,530 --> 00:04:21,220
There is a ton of stuff in there, and

80
00:04:21,220 --> 00:04:23,580
there's no way I can cover
it all in a single course.

81
00:04:23,580 --> 00:04:25,140
There's a link to the docs
in the teacher's notes.

82
00:04:26,190 --> 00:04:29,950
Now that you're comfortable with what a
path is let's see how Python can construct

83
00:04:29,950 --> 00:04:31,210
paths for us.

84
00:04:31,210 --> 00:04:33,289
This will help us avoid having
to use that double backslash or

85
00:04:33,289 --> 00:04:35,104
pay attention to which
direction our slashes face.