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- [Instructor] In this video,

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let's talk about updating text files.

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Now, let's assume that we have
a record like 300 White 0.00

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and what we wanna do in this case

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is update the name White
to the name Williams.

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Now clearly Williams has
more characters than White,

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so in a typical file updating scenario,

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you would locate the
information that you want

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and then you would overwrite
the existing information

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but you can only do that if every record

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has fixed length fields.

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So, for example, in a basic text file,

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if I were to replace White with Williams,

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I would actually wind
up overwriting a portion

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of the next field of data
in the original record.

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So, the typical way that
you'll update a text file

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is by creating a temporary file,

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reading all of the records
before the information

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you wanna update and writing them

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into the temporary file,

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writing the updated record
into the temporary file,

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writing the rest of the original file

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into the temporary file

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and then using the temporary file

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to overwrite the original.

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So, that's what I'd like to demonstrate

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in this video.

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So, let's go ahead and switch
over to a terminal Window

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and for this purpose,

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because we're going to need both

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the original accounts.txt file

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that we've been working with,

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and a temporary file,

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I'm actually going to create a couple

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of statements here that
open up those files

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and now we can still use these files

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in a with statement and
still get the benefit

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of closing those files
automatically in a with statement

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which is also what I'd
like to talk about here.

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So, first thing we're going to do

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is open the accounts.txt file

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and we're going to open it for reading

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and separately, we're going
to create a temporary file

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called temp_file.txt

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and we're going to open
that one for writing.

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Now let me go ahead and
copy a with statement,

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so we can talk about how the update

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is going to occur.

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So, as you can see here,

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if you want to have more
than one resource managed

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by a with statement,

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you can specify them in
a comma-separated list

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of values, in this case,

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the variable names that
we created previously

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in the upper two snippets,
snippets one and two

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but we could have put
these open expressions

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into the with statement

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if we had wanted to do that.

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In that case, we would have
said with open accounts.txt

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for reading as accounts

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and then we would have had a comma

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and then open the next file

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as temp_file and then the
colon for the with statements,

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to introduce the with
statement's suite, excuse me.

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So, now we have the two files

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that we're going to manage

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and we are going to use this for loop

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to perform the process that
I overviewed previously.

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For every record in the accounts file,

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we're going to break
apart the current record

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into its account name and balance.

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We are going to check whether
the specific account number

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is 300 because we want to
update White to Williams,

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so if indeed the account
number is not 300,

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we will simply write the
current full record out

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to the temporary file,

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otherwise we will create a new record

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of information that we will write

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into the temporary file.

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So, as you can see, what we're doing here

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is joining with the space character

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as the separator a list

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of values that have
the same account number

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that we had previously,

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the new last name

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and the balance,

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so this will create the new record string

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and then we'll simply
write that new record

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followed by a new line out to the file,

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so of course when we
execute that with statement,

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it's going to perform the task

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that we just reviewed,

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it's then going to close the accounts

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and temp_file files.

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Now at this point, we
have two files on disk,

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the original accounts file

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and the temp_file, so for example,

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again, I can list out the contents

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of those files, so I'll
go ahead and again,

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if you're a Windows user,

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put the word more here instead of cat,

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so this is the original file,

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it still has White in it

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and of course if I go ahead
and bring up my temp_file,

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you can see it's got Williams

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which is the correct information,

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so now what we wanna do is
have this file overwrite

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the proceeding file

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and then we want to
discard the temporary file

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of information, so for this purpose,

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we're going to take advantage

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of the Python standard library's
built-in module called os

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which is for operating system commands

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and the first thing we're going to do

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is actually get rid of
the accounts.txt file

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and then we're going to rename

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the temporary file to accounts.txt.

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So, there's a function called remove

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in the os module, you give it the name

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of the file to remove and again,

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if you don't specify path information,

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it's assuming that the file
is in the current folder

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from which we're working,

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so now we've deleted accounts.txt

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and I can go ahead and
list out the contents

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or if you're a Windows user,

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do a dir instead

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and you can see there's no
text file named accounts.txt

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in the current folder anymore.

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Then I can go ahead

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and rename the temporary file

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using the rename function
from the os module,

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so here's the file that we want to rename,

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here is the new name for that file.

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Now if I list out the directory contents

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and again Windows users should use dir,

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you can see indeed we have accounts.txt

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and we no longer have
the temporary file listed

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in the current folder,

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so at this point, we can go ahead

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and list out the contents
of the accounts.txt file

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and indeed it has the updated last name

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for the record 300 that
we had modified up above.