Community Data Science Workshops (Spring 2014)/Friday April 4th Tutorial: Difference between revisions

There's a helpful <b>'''function</b>''' (more on what a function is in a second) called <code>type</code> that tells you what kind of thing -- what <b>'''data type</b>''' -- Python thinks something is. We can check for ourselves that Python considers '1' and '1.0' to be different data types:

So now we've seen two data types: <b>'''integers</b>''' and <b>'''floats</b>'''.

* To use a function, write the name of the function, followed by an open parenthesis, then what the function needs as input (we call that input the <b>'''arguments</b>''' to the function), and then a close parenthesis.

Stop here and try hitting the Up arrow on your keyboard a few times. The Python <b>'''interpreter</b>''' saves a history of what you've entered, so you can arrow up to old commands and hit Return to re-run them!

Giving a name to something, so that you can refer to it by that name, is called <b>'''assignment</b>'''. Above, we assigned the name 'x' to 4, and after that we can use <code>x</code> wherever we want to use the number 4.

So far we've seen two data types: <b>'''integers</b>''' and <b>'''floats</b>'''. Another useful data type is a <b>'''string</b>''', which is just what Python calls a bunch of characters (like numbers, letters, whitespace, and punctuation) put together. Strings are indicated by being surrounded by quotes:

Python is giving us a <b>'''traceback</b>'''. A traceback is details on what was happening when Python encountered an Exception or Error -- something it doesn't know how to handle.

This gives us another <b>'''traceback</b>''', for a new kind of error, a <code>SyntaxError</code>. When Python looks at that expression, it sees the string 'I' and then

So far we've been learning at the interactive <b>'''Python interpreter</b>'''. When you are working at the interpreter, any work that you do gets printed to the screen. For example:

Another place that we will be writing Python code is in a file. When we run Python code from a file instead of interactively, we don't get work printed to the screen for free. We have to tell Python to print the information to the screen. The way we do this is with the <b>'''print</b>''' function. Here's how it works:

The string manipulate is exactly the same as before. The only difference is that you need to use <b>'''print</b>''' to print results to the screen:

part, and press Enter. The next line will have <code>...</code> as a prompt, instead of the usual <code>&gt;&gt;&gt;</code>. This is Python telling us that we are in the middle of a <b>'''code block</b>''', and so long as we indent our code it should be a part of this code block.</li>

What is going on here? When Python encounters the <code>if</code> keyword, it <i>evaluates</i> the <i>expression</i> following the keyword and before the colon. If that expression is <b>'''True</b>''', Python executes the code in the indented code block under the <code>if</code> line. If that expression is <b>'''False</b>''', Python skips over the code block.

<b>'''<code>if</code></b>''' lets you execute some code only if a condition is <code>True</code>. What if you want to execute different code if a condition is <code>False</code>?

Use the <b>'''<code>else</code></b>''' keyword, together with <code>if</code>, to execute different code when the <code>if</code> condition isn't <code>True</code>. Try this:

You can check multiple expressions together using the <b>'''<code>and</code></b>''' and <b>'''<code>or</code></b>''' keywords. If two expressions are joined by an <code>and</code>, they <b>'''both</b>''' have to be <code>True</code> for the overall expression to be <code>True</code>. If two expressions are joined by an <code>or</code>, as long as <b>'''at least one</b>''' is <code>True</code>, the overall expression is <code>True</code>.

If you need to execute code conditional based on more than two cases, you can use the <b>'''<code>elif</code></b>''' keyword to check more cases. You can have as many <code>elif</code> cases as you want; Python will go down the code checking each <code>elif</code> until it finds a <code>True</code> condition or reaches the default <code>else</code> block.

<b>'''Remember that '=' is for assignment and '==' is for comparison.</b>'''

So far, the code we've written has been <i>unconditional</i>: no choice is getting made, and the code is always run. Python has another data type called a <b>'''boolean</b>''' that is helpful for writing code that makes decisions. There are two booleans: <code>True</code> and <code>False</code>.

This is an important idea and can be a source of bugs until you get used to it: <b>'''= is assignment, == is comparison</b>'''.