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- Welcome to lesson 9,
files and exceptions.

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In this lesson, we're
going to look at a number

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of different file processing techniques,

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and also introduce the Python
exception handling mechanism

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for dealing with problems as
they occur at execution time.

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And the reason we bundled these
two things together is that

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in the context of file processing,

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you will often run in
to various exceptions,

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so it just felt like a good
way to introduce both topics.

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Now, we'll start out in this lesson

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with some basic text-file processing

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and we'll use similar techniques to work

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with what's known as
JavaScript Object Notation,

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and that which we call JSON for short is

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one of the most popular
data interchange formats

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for sending and receiving
data over the Internet.

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So we'll demonstrate
a library called json,

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a module called json that's built

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into the Python standard library.

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And in both the text-file processing

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and the JSON file
processing we will be using

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a Python statement with, which enables you

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to acquire a resource such as a file

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that you open a connection
to on your disk,

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use that resource and then
automatically de-allocate

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or close that resource
when you're done using it,

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so with statements are
going to be frequently used

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any time you do things like opening files

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for reading or writing,
opening database connections

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to read from a database
or write into a database,

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opening network connections to send bytes

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and characters over the Internet,

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or receive them onto your machine,

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and many other similar scenarios, as well.

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So after that, we're going to move onto

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the exception handling part of the lesson,

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and there we'll introduce
the various syntax elements

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for exception handling in Python.

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And part of what I'll do
there is compare and contrast

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how Python does things
with some of the other

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C-based languages, specifically
C++, Java and C# as well.

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You'll see a lot of similarities,

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but you will also see some
differences in Python's way

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of doing things with the
exception handling mechanism.

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As part of that we'll
introduce the try statement.

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It has instead of a catch
clause, it has an except clause,

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unlike some of the other
programming languages.

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We'll also introduce the else clause

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which none of the other
programming languages has.

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And the finally clause, as well,

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which you may or may not
know from Java and C#.

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Also in the exception
handling presentation,

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we'll take a look at the raise statement,

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which is how you throw
an exception in Python.

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We call that raising
an exception in Python.

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And we'll take a look in
more detail at what's known

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as a traceback in the context of Python.

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Other programming languages
typically refer to that

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as a stack trace.

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Python has some different
information in the traceback

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and it's ordered differently
compared to a lot

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of the other programming languages
you may be familiar with.

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Now, once we get to the intro

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to data science section in this lesson,

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we'll go back to our
discussions of file processing,

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this time in the context
of a library called csv,

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which stands for comma-separated values

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that's built into the
Python standard library.

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And one of the reasons for that is

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from this point forward
we're going to do a lot

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with data sets in subsequent lessons

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and a lot of those data sets are provided

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in comma-separated value format,

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so it is important to understand
how to read that format,

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also how to write that format as well.

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And to finish up that section,

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we're also going to do that in the context

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of the pandas library,

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which it turns out provides
a super simple function

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that you can use to load data

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from a data set represented as a CSV file.