Python for Absolute Beginners Transcripts
Chapter: Reading and writing files in Python
Lecture: Demo: Safer file handling
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Loadings the rolls worked great
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but there's a few things that we need to work on.
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First of all, notice this.
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Now this is a topic we've not covered at all
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we've experienced it, but we haven't covered it.
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If there's a problem with some code in Python
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what happens is Python throws what's called an Exception
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and that means if you're coming down here, and here
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and like right on this line there's an Exception
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it's not like this comes back as nothing.
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It's going to stop what it's doing
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figure out if there's some kind of error handling block
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it's not here, so it's going to go see who called this
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and it's just going to leave
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it's just going to stop on line 95 and carry on.
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You know, maybe worse
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if so the error happened on 36, or 96
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the file's going to be open, and we're going to leave
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and that could be a big problem.
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So we want to be safer about that.
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We want to make sure that no matter what
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even if there's an error
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we call this close function.
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So we can, there's a different way to write this
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these three lines of code here.
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Check this out. This is a new construct, it's called a Context Manager.
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It uses the 'with' keyword
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so you say 'with' whatever we had here
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as whatever the variable name is, so fin
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all right take that, put it there
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and then we indent into this block here.
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Whatever happens in this block
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is going to be when the file is open
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and then when we leave
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you can't really see it here, let me put some more
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you look really carefully see
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there's a vertical line right there?
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This is PyCharm hinting
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oh all this stuff is inside the block.
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Whenever in the with block
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whenever we leave this
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and we go back to, like the next thing is print
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it's going to automatically call close.
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And importantly, it's going to that even
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if there is an error.
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So that, that bit right there
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is a safer version of this
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cause this doesn't check if there is an error
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and also call close.
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That would take like three or four more lines of code.
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So we're not going to do that.
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We're just going to do this.
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Whenever your working on files
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always use a with block there.
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I'll leave this in as a comment for you
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but we're not going to run with that.
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Just one more time, make sure it works.
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Yes, it loaded it up all fine.
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Perfect!
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So, everything is looking great
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this with block, this context manager
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is recommended any time you have something
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that you must must must close, clean up
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blush, shut down
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whatever as part of you've opened it
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do some stuff, and now close it
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even if there is an error.
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This is common in databases
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if you're writing to a database
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you need to have a transaction
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you want to either commit or roll it back
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if there's an error. So it appears in many many places
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also networks and socket connections
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but files is one of the places
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you'll definitely run into these context managers
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so don't forgot to use them.
