Boston Python workshop 2/Friday tutorial: Difference between revisions

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<pre>
<pre>
if 1:
if True:
print "1 is truthy"
print "I'm True!"
</pre>
</pre>


That was our first multi-line piece of code, and the way to enter it at a Python prompt is a little different. First, type the <code>if 1:</code> part, and hit 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.
That was our first multi-line piece of code, and the way to enter it at a Python prompt is a little different. First, type the <code>if True:</code> part, and hit 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.


Type 4 spaces, and then type <code>print "1 is truthy"</code>. Hit enter to end the line, and hit enter again to tell Python you are done with this code block. All together, it will look something like this:
Type 4 spaces, and then type <code>print "I'm True!"</code>. Hit enter to end the line, and hit enter again to tell Python you are done with this code block. All together, it will look something like this:


<pre>
<pre>
>>> if 1:
>>> if True:
... print "1 is truthy"
... print "I'm True!"
...
...
"I'm True!"
⚫
1 is truthy
</pre>
</pre>


So 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.
So 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.


In this case, because <code>1</code> is truthy, Python executes the code block under the if statement, and we see "1 is truthy" printed to the screen.
In this case, because <code>1</code> is truthy, Python executes the code block under the if statement, and we see "I'm True!" printed to the screen. What do you think will happen here?


<pre>
if False:
print "I'm True!"
</pre>


<pre>
What about a 0? Is it truthy?
if 1:
⚫
print "1 is truthy"
</pre>


<pre>
<pre>
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