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If you write to a file, there's a really good chance you're going to want to read from that file. Here's how to read in the contents of a file in Python.
open(filename, mode="r")opens a file. More info in the docs.
file.read(bytes=-1) would read the entire contents of the file. You can control the number of bytes read by passing in an integer. Relatedly, file.seek() will move the read/write pointer to another part of the file.
file.readlines() reads the entire file into a list, with each line as a list item.
The context manager pattern for dealing with files is:
with open("my_file.txt", "r") as file:
file.read(10)
For more about sys.argv, check out the docs.
So I've changed the script a little bit.
0:00
I've imported sys which
I'm using down here.
0:03
I'll talk about what argv
does in just a minute.
0:05
And I've added this function called show.
0:08
This is what we are going to use to
print out the things that we've saved.
0:10
What is sys.argv?
0:14
Sys.argv is all of the arguments
after your filename.
0:18
So if I do python remember.py,
and I pass in and
0:23
juice boxes that's a common
shopping thing for
0:29
me, and I come over here to the database,
we'll see juice boxes added in.
0:35
Juice boxes here is what's in argv.
0:38
This is argv zero.
0:42
This is argv one.
0:45
And this is argv two and so on.
0:46
So, when I do this space.join.sys.argv.1:
0:49
that grabs everything after remember.py
and joins it together with the space.
0:54
So that's how we can take all those words.
0:59
What I'm doing is I'm checking to
see if the first word is --list.
1:01
And if it is,
then I want to run this show function.
1:05
But show currently doesn't do anything.
1:08
So let's fill that out, make it do stuff.
1:11
All right, so we've already talked
about how to use a context manager.
1:13
So let's do that.
1:19
So we'll say with open("database.txt",
1:22
and now I can do r but that's the default
so if I don't specify it, it still does r.
1:28
And, we'll call if file, and
1:34
then inside of here we wanna print
out all the stuff that's in the file.
1:36
We just have the file open.
1:41
So, how do we print out
everything that's in that file?
1:43
Well, the cool this is, Python treats
files kind of like they are iterables.
1:45
They are iterables, but it treats them
like they're a list or something.
1:51
So I can do for
line in file: print(line) all right,
1:54
and I don't need to close the file because
the context processor handles that for me.
2:01
So let's save that and so
lets do python remember.py --list.
2:08
And check that out,
there's all of our stuff.
2:14
Let's run this one more time.
2:18
There we go,
there's our stuff that we wanted to get.
2:20
Orange Juice, milk, a sweater,
a sword, and juice boxes.
2:22
I'm hopeful I can go to a store that
has all of those things at once.
2:25
So cool.
2:30
There's all of our stuff.
2:31
I have two other methods that I can use
to control how stuff is read though.
2:33
Instead of writing these into our file,
let's just play with them in the terminal.
2:38
So I'm gonna actually slide that most
of the way up, and I'm gonna clear, and
2:42
I'm gonna go into Python.
2:46
So, I have a file that has all the
teachers in it, it's called teachers.txt.
2:49
And I'm gonna mess with that one.
2:53
So I'm gonna open it up.
2:56
open('teachers.txt').
2:57
I could pass in the r flag for
reading it, but
3:01
I don't need to do that
because it's read by default.
3:03
And let's do file.read(10).
3:07
I wanna read 10 bytes of this file.
3:09
And the 10 is the number of bytes and
I get Nick Petti, so, cool new name, Nick.
3:14
And so, the cool thing is that this
lets me just read parts of a file.
3:20
If I know that I can only process,
say, 1,024 bytes at a time,
3:24
then I can read in just
1,024 bytes at a time.
3:29
So, that's pretty cool.
3:33
It's pretty handy.
3:34
And you can read the entire
file if you want,
3:35
by doing it with no argument or
with a negative number.
3:38
If we do file.read, we get the rest of the
file and it's the rest of the file because
3:44
here's the t that should
have been on Nick's name.
3:48
It's not because when you
have a file object in Python,
3:52
there is a little argument,
a little value, on that file object.
3:56
That's a pointer to where
we've read to in the file.
4:00
We know how much we read up to.
4:04
So we know that we've read 10 bytes.
4:06
So when we say, okay,
keep reading that file,
4:09
it starts at the 11th byte and
keeps moving.
4:12
The way that we reset that,
is we can use file.seek.
4:15
And if we just do file.seek
it has to have an argument so
4:19
we tell it where to go and
we say go back to 0.
4:22
And it gives us back
where it currently is.
4:25
It's currently at 0.
4:27
All right, so that's cool.
4:29
And then you do file.read(15) and we've
got the 11 characters of Nick's name,
4:30
the one new line character, and
then three characters of Amit's name.
4:37
That adds up to 15 bytes total.
4:41
All right, that's really handy.
4:44
I want to show you one last thing
about reading from files though.
4:45
Let's go back to the beginning and
lots of times we have a file and
4:50
it has a whole lot of lines in it and
each line needs to be a thing on its own.
4:53
So, the way that we can do that is
by using the read-lines method.
4:58
There's also a read-line method,
which would just read one line.
5:02
But read-lines is way more useful.
5:06
So let's do lines=file.readlines and
then let's look at the len of the lines.
5:09
So cool,
it says there's 17 things in there.
5:17
All right, let's look at our teachers.txt
and yep, there are 17 lines.
5:20
And the cool thing is we could do like,
for line in lines:
5:27
print(line[::-1]).
5:32
And there's everybody's names backwards.
5:38
So that's kind of cool.
5:40
There's lots of other things you can
do with files so go and explore them.
5:42
Using files for
storing data is a really handy thing for
5:48
a lot of programming problems.
5:51
It's also a great way to
get output from a script
5:52
if you need the script to run in
the background or something like that.
5:54
You might want to use the logging
library for that though, so
5:58
be sure to check it out before jumping
straight to writing to files yourself.
6:00
If you're working with files, there's
a good chance you're going to need to use
6:04
CRV or JSON files or some other format.
6:07
Keep an eye out for another workshop
about both of those file formats.
6:10
As always, if you have an idea for
future workshops, let me know.
6:14
Thanks for watching and I'll see you soon
6:17
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