Community Data Science Workshops (Spring 2015)/Day 0 tutorial: Difference between revisions

From CommunityData
(copied content from Fall 2014 Day 0 tutorial)
(cat)
 
(8 intermediate revisions by 3 users not shown)
Line 5: Line 5:
This is an interactive tutorial! As you go through this tutorial, any time you see something that looks like this:
This is an interactive tutorial! As you go through this tutorial, any time you see something that looks like this:


<syntaxhighlight lang="python">
  a = "Hello"
  a = "Hello"
</syntaxhighlight>


you should type the expression at a '''Python''' prompt, hitting Return after every line and noting the output.
you should type the expression at a '''Python''' prompt, hitting Return after every line and noting the output.
Line 19: Line 21:
===Addition===
===Addition===


<syntaxhighlight lang="python">
  2 + 2
  2 + 2
  1.5 + 2.25
  1.5 + 2.25
 
</syntaxhighlight>
===Subtraction===
===Subtraction===
 
<syntaxhighlight lang="python">
  4 - 2
  4 - 2
  100 - .5
  100 - .5
  0 - 2
  0 - 2
 
</syntaxhighlight>
===Multiplication===
===Multiplication===
 
<syntaxhighlight lang="python">
  2 * 3
  2 * 3
</syntaxhighlight>
===Division===


===Division===
First, lets try some basic fractions:


4 / 2
<syntaxhighlight lang="python">
  1 / 2
  1 / 2
</syntaxhighlight>
Hey now! That last result is probably not what you expected. What's going on here is that integer divison produces an integer. You need a number that knows about the decimal point to get a decimal out of division:


1.0 / 2
Sure enough, Python says that the answer is 0.5 which is the same as 1/2.


This means you have to be careful when manipulating fractions. If you were doing some baking and needed to add 3/4 of a cup of flour and 1/4 of a cup of flour, we know in our heads that 3/4 + 1/4 = 1 cup. But try that at the Python prompt:
Let's try the same thing:


  3/4 + 1/4
<syntaxhighlight lang="python">
  4 / 2
</syntaxhighlight>


What do you need to do to get the right answer? Use data types that understand decimals for each of the divisions:
Hey now! That last result is a little strange. When you divide a number in Python, even if the answer doesn't need a decimal place (like <code>4/2</code> which equals 2), you get an answer with a decimal point!


3.0/4 + 1.0/4
What's going on here is that in Python, division produces an what's called a <code>float</code> which essentially means a number with a decimal point.
3.0/4.0 + 1.0/4.0


The two previous expressions produce the same result. You only need to make one of the numbers in each fraction have a decimal. When the Python interpreter goes to do the division, it notices that one of the numbers in the fraction cares about decimals and says "that means I have to make the other number care about decimals too".
When the Python interpreter goes to do the division, it knows that (unlike multiplication for example) division can lead to numbers that aren't whole numbers (like <code>1/2</code>) so it makes sure that the result will always include a decimal place just in case it's needed.
 
 
This shouldn't be important for this workshop but it might be worth knowing that older versions of Python (before version 3) would always round down and return integers instead of giving numbers with decimal places.


==Types==
==Types==
Line 57: Line 65:


There's a helpful '''function''' (more on what a function is in a second) called <code>type</code> that tells you what kind of thing -- what '''data type''' -- Python thinks something is. We can check for ourselves that Python considers '1' and '1.0' to be different data types:
There's a helpful '''function''' (more on what a function is in a second) called <code>type</code> that tells you what kind of thing -- what '''data type''' -- Python thinks something is. We can check for ourselves that Python considers '1' and '1.0' to be different data types:
 
<syntaxhighlight lang="python">
  type(1)
  type(1)
  type(1.0)
  type(1.0)
</syntaxhighlight>
So now we've seen two data types: '''integers''' and '''floats'''.
So now we've seen two data types: '''integers''' and '''floats'''.


Line 85: Line 93:


A lot of work gets done in Python using variables. Variables are a lot like the variables in math class, except that in Python variables can be of any data type, not just numbers.
A lot of work gets done in Python using variables. Variables are a lot like the variables in math class, except that in Python variables can be of any data type, not just numbers.
 
<syntaxhighlight lang="python">
  type(4)
  type(4)
  x = 4
  x = 4
Line 91: Line 99:
  type(x)
  type(x)
  2 * x
  2 * x
 
</syntaxhighlight>
Giving a name to something, so that you can refer to it by that name, is called '''assignment'''. 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.
Giving a name to something, so that you can refer to it by that name, is called '''assignment'''. 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.


Variables can't have spaces or other special characters, and they need to start with a letter. Here are some valid variable names:
Variables can't have spaces or other special characters, and they need to start with a letter. Here are some valid variable names:
 
<syntaxhighlight lang="python">
<code>magic_number = 1500</code><br />
magic_number = 1500
<code>amountOfFlour = .75</code><br />
amountOfFlour = .75
<code>my_name = "Jessica"</code>
my_name = "Jessica"
</syntaxhighlight>


Projects develop naming conventions: maybe multi-word variable names use underscores (like <code>magic_number</code>), or "camel case" (like <code>amountOfFlour</code>). The most
Projects develop naming conventions: maybe multi-word variable names use underscores (like <code>magic_number</code>), or "camel case" (like <code>amountOfFlour</code>). The most
Line 109: Line 118:
Notice how if you type a 4 and hit enter, the Python interpreter spits a 4 back
Notice how if you type a 4 and hit enter, the Python interpreter spits a 4 back
out:
out:
 
<syntaxhighlight lang="python">
  4
  4
 
</syntaxhighlight>
But if you assign 4 to a variable, nothing is printed:
But if you assign 4 to a variable, nothing is printed:
 
<syntaxhighlight lang="python">
  x = 4
  x = 4
 
</syntaxhighlight>
You can think of it as that something needs to get the output. Without an assignment, the winner is the screen. With assignment, the output goes to the variable.
You can think of it as that something needs to get the output. Without an assignment, the winner is the screen. With assignment, the output goes to the variable.


You can reassign variables if you want:
You can reassign variables if you want:
 
<syntaxhighlight lang="python">
  x = 4
  x = 4
  x
  x
  x = 5
  x = 5
  x
  x
 
</syntaxhighlight>
Sometimes reassigning a variable is an accident and causes bugs in programs.
Sometimes reassigning a variable is an accident and causes bugs in programs.
 
<syntaxhighlight lang="python">
  x = 3
  x = 3
  y = 4
  y = 4
Line 132: Line 141:
  x * x
  x * x
  2 * x - 1 * y
  2 * x - 1 * y
 
</syntaxhighlight>
Order of operations works pretty much like how you learned in school. If you're unsure of an ordering, you can add parentheses like on a calculator:
Order of operations works pretty much like how you learned in school. If you're unsure of an ordering, you can add parentheses like on a calculator:
 
<syntaxhighlight lang="python">
  (2 * x) - (1 * y)
  (2 * x) - (1 * y)
 
</syntaxhighlight>
Note that the spacing doesn't matter:
Note that the spacing doesn't matter:
 
<syntaxhighlight lang="python">
  x = 4
  x = 4
 
</syntaxhighlight>
and
and
 
<syntaxhighlight lang="python">
  x=4
  x=4
 
</syntaxhighlight>
are both valid Python and mean the same thing.
are both valid Python and mean the same thing.
 
<syntaxhighlight lang="python">
  (2 * x) - (1 * y)
  (2 * x) - (1 * y)
 
</syntaxhighlight>
and
and
 
<syntaxhighlight lang="python">
  (2*x)-(1*y)
  (2*x)-(1*y)
 
</syntaxhighlight>
are also both valid and mean the same thing. You should strive to be consistent with whatever spacing you like or a job requires, since it makes reading the code easier.
are also both valid and mean the same thing. You should strive to be consistent with whatever spacing you like or a job requires, since it makes reading the code easier.


Line 162: Line 171:


So far we've seen two data types: '''integers''' and '''floats'''. Another useful data type is a '''string''', 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:
So far we've seen two data types: '''integers''' and '''floats'''. Another useful data type is a '''string''', 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:
 
<syntaxhighlight lang="python">
  "Hello"
  "Hello"
  "Python, I'm your #1 fan!"
  "Python, I'm your #1 fan!"
 
</syntaxhighlight>
Like with the math data types above, we can use the <code>type</code> function to check the type of strings:
Like with the math data types above, we can use the <code>type</code> function to check the type of strings:
 
<syntaxhighlight lang="python">
  type("Hello")
  type("Hello")
  type(1)
  type(1)
  type("1")
  type("1")
 
</syntaxhighlight>
===String Concatenation===
===String Concatenation===


You can smoosh strings together (called "concatenation") using the '+' sign:
You can smoosh strings together (called "concatenation") using the '+' sign:
 
<syntaxhighlight lang="python">
  "Hello" + "World"
  "Hello" + "World"


  name = "Jessica"
  name = "Jessica"
  "Hello " + name
  "Hello " + name
 
</syntaxhighlight>
How about concatenating different data types?
How about concatenating different data types?
 
<syntaxhighlight lang="python">
  "Hello" + 1
  "Hello" + 1
 
</syntaxhighlight>
Hey now! The output from the previous example was really different and interesting; let's break down exactly what happened:
Hey now! The output from the previous example was really different and interesting; let's break down exactly what happened:


<code>>>> "Hello" + 1</code><br />
<pre>
<code>Traceback (most recent call last):</code><br />
>>> "Hello" + 1
<code>  File "<stdin>", line 1, in <module></code><br />
Traceback (most recent call last):
<code>TypeError: cannot concatenate 'str' and 'int' objects</code>
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects
</pre>


Python is giving us a '''traceback'''. A traceback is details on what was happening when Python encountered an Exception or Error -- something it doesn't know how to handle.
Python is giving us a '''traceback'''. A traceback is details on what was happening when Python encountered an Exception or Error -- something it doesn't know how to handle.
Line 222: Line 233:
There's another useful function that works on strings called <code>len</code>. <code>len</code> returns the length of a string as an integer:
There's another useful function that works on strings called <code>len</code>. <code>len</code> returns the length of a string as an integer:


<syntaxhighlight lang="python">
  len("Hello")
  len("Hello")
  len("")
  len("")
Line 227: Line 239:
  name_length = len(fish)
  name_length = len(fish)
  fish + " is a Hawaiian fish whose name is " + str(name_length) + " characters long."
  fish + " is a Hawaiian fish whose name is " + str(name_length) + " characters long."
</syntaxhighlight>


===Quotes===
===Quotes===


We've been using double quotes around our strings, but you can use either double or single quotes:
We've been using double quotes around our strings, but you can use either double or single quotes:
 
<syntaxhighlight lang="python">
  'Hello'
  'Hello'
  "Hello"
  "Hello"
 
</syntaxhighlight>
Like with spacing above, use whichever quotes make the most sense for you, but be consistent.
Like with spacing above, use whichever quotes make the most sense for you, but be consistent.


Line 252: Line 265:


One fun thing about strings in Python is that you can multiply them:
One fun thing about strings in Python is that you can multiply them:
 
<syntaxhighlight lang="python">
  "A" * 40
  "A" * 40
  "ABC" * 12
  "ABC" * 12
Line 258: Line 271:
  b = "Birthday"
  b = "Birthday"
  (h + b) * 10
  (h + b) * 10
 
</syntaxhighlight>
==Part 1 Practice==
==Part 1 Practice==


Line 266: Line 279:


1.
1.
 
<syntaxhighlight lang="python">
  total = 1.5 - 1/2
  total = 1.5 - 1/2
  total
  total
  type(total)
  type(total)
 
</syntaxhighlight>


2.
2.
 
<syntaxhighlight lang="python">
  a = "quick"
  a = "quick"
  b =  "brown"
  b =  "brown"
  c = "fox jumps over the lazy dog"
  c = "fox jumps over the lazy dog"
  "The " +  a * 3 + " " +  b * 3 + " " + c
  "The " +  a * 3 + " " +  b * 3 + " " + c
 
</syntaxhighlight>
==End of Part 1==
==End of Part 1==


Line 290: Line 303:


So far we've been learning at the interactive '''Python interpreter'''. When you are working at the interpreter, any work that you do gets printed to the screen. For example:
So far we've been learning at the interactive '''Python interpreter'''. When you are working at the interpreter, any work that you do gets printed to the screen. For example:
 
<syntaxhighlight lang="python">
  h = "Hello"
  h = "Hello"
  w = "World"
  w = "World"
  h + w
  h + w
 
</syntaxhighlight>
will display "HelloWorld".
will display "HelloWorld".


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 '''print''' function. Here's how it works:
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 '''print''' function. Here's how it works:
 
<syntaxhighlight lang="python">
  h = "Hello"
  h = "Hello"
  w = "World"
  w = "World"
Line 305: Line 318:
  my_string = "Alpha " + "Beta " + "Gamma " + "Delta"
  my_string = "Alpha " + "Beta " + "Gamma " + "Delta"
  print(my_string)
  print(my_string)
 
</syntaxhighlight>


The string manipulate is exactly the same as before. The only difference is that you need to use '''print''' to print results to the screen:
The string manipulate is exactly the same as before. The only difference is that you need to use '''print''' to print results to the screen:
Line 324: Line 337:


<ol>
<ol>
<li>Download the file http://mako.cc/teaching/2014/cdsw/nobel.py by right-clicking on it and saying to save it as a ".py" file to your Desktop. The ".py" extension hints that this is a Python script.</li>
<li>Download the file http://mako.cc/teaching/2015/cdsw-spring/nobel.py by right-clicking on it and saying to save it as a ".py" file to your Desktop. The ".py" extension hints that this is a Python script.</li>
<li>Open a terminal prompt, and use the navigation commands (<code>dir</code> and <code>cd</code> on Windows, <code>ls</code>, <code>pwd</code>, and <code>cd</code> on OS X and Linux) to navigate to your Desktop directory. See [[Community Data Science Workshops (Fall 2014)/Day 0 setup and tutorial#Goal_.234:_practice_navigating_the_computer_from_a_terminal|navigating from a terminal]] for a refresher on those commands.</li>
<li>Open a terminal prompt, and use the navigation commands (<code>dir</code> and <code>cd</code> on Windows, <code>ls</code>, <code>pwd</code>, and <code>cd</code> on OS X and Linux) to navigate to your Desktop directory. See [[Community_Data_Science_Workshops_(Spring_2015)/Day_0_setup_and_tutorial#Goal_.234:_Practice_navigating_the_computer_from_a_terminal|navigating from a terminal]] for a refresher on those commands.</li>
<li>Once you are in your Desktop directory, execute the contents of <code>nobel.py</code> by typing
<li>Once you are in your Desktop directory, execute the contents of <code>nobel.py</code> by typing


Line 334: Line 347:
<code>nobel.py</code> introduces two new concepts: comments and multiline strings.</li>
<code>nobel.py</code> introduces two new concepts: comments and multiline strings.</li>


<li>Open <code>nobel.py</code> in your text editor (see [[Community Data Science Workshops (Fall 2014)/Day 0 setup and tutorial#Goal_.232:_prepare_a_text_editor|preparing your text editor]] for a refresher on starting the editor).</li>
<li>Open <code>nobel.py</code> in your text editor (see [[Community_Data_Science_Workshops_(Spring_2015)/Day_0_setup_and_tutorial#Goal_.232:_Prepare_a_text_editor|preparing your text editor]] for a refresher on starting the editor).</li>
<li>Read through the file in your text editor carefully and check your understanding of both the comments and the code.</li>
<li>Read through the file in your text editor carefully and check your understanding of both the comments and the code.</li>
</ol>
</ol>
Line 404: Line 417:
Use the '''<code>else</code>''' keyword, together with <code>if</code>, to execute different code when the <code>if</code> condition isn't <code>True</code>. Try this:
Use the '''<code>else</code>''' keyword, together with <code>if</code>, to execute different code when the <code>if</code> condition isn't <code>True</code>. Try this:


<pre>
<syntaxhighlight lang="python">
sister_age = 15
sister_age = 15
brother_age = 12
brother_age = 12
Line 411: Line 424:
else:
else:
     print("brother is older")
     print("brother is older")
</pre>
</syntaxhighlight>


Like with <code>if</code>, the code block under the <code>else</code> condition must be indented so Python knows that it is a part of the <code>else</code> block.
Like with <code>if</code>, the code block under the <code>else</code> condition must be indented so Python knows that it is a part of the <code>else</code> block.
Line 421: Line 434:
Try typing these out and see what you get:
Try typing these out and see what you get:


<pre>
<syntaxhighlight lang="python">
1 > 0 and 1 < 2
1 > 0 and 1 < 2
</pre>
</syntaxhighlight>


<pre>
<syntaxhighlight lang="python">
1 < 2 and "x" in "abc"
1 < 2 and "x" in "abc"
</pre>
</syntaxhighlight>


<pre>
<syntaxhighlight lang="python">
"a" in "hello" or "e" in "hello"
"a" in "hello" or "e" in "hello"
</pre>
</syntaxhighlight>


<pre>
<syntaxhighlight lang="python">
1 <= 0 or "a" not in "abc"
1 <= 0 or "a" not in "abc"
</pre>
</syntaxhighlight>


Guess what will happen when you enter these next two examples, and then type them out and see if you are correct. If you have trouble with the indenting, call over a staff member and practice together. It is important to be comfortable with indenting for tomorrow.
Guess what will happen when you enter these next two examples, and then type them out and see if you are correct. If you have trouble with the indenting, call over a staff member and practice together. It is important to be comfortable with indenting for tomorrow.


<pre>
<syntaxhighlight lang="python">
temperature = 32
temperature = 32
if temperature > 60 and temperature < 75:
if temperature > 60 and temperature < 75:
Line 445: Line 458:
else:
else:
     print("Too extreme for me.")
     print("Too extreme for me.")
</pre>
</syntaxhighlight>


<pre>
<syntaxhighlight lang="python">
hour = 11
hour = 11
if hour < 7 or hour > 23:
if hour < 7 or hour > 23:
Line 455: Line 468:
     print("Welcome to the cheese shop!")
     print("Welcome to the cheese shop!")
     print("Can I interest you in some choice gouda?")
     print("Can I interest you in some choice gouda?")
</pre>
</syntaxhighlight>


You can have as many lines of code as you want in <code>if</code> and <code>else</code> blocks; just make sure to indent them so Python knows they are a part of the block.
You can have as many lines of code as you want in <code>if</code> and <code>else</code> blocks; just make sure to indent them so Python knows they are a part of the block.
Line 463: Line 476:
If you need to execute code conditional based on more than two cases, you can use the '''<code>elif</code>''' 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.
If you need to execute code conditional based on more than two cases, you can use the '''<code>elif</code>''' 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.


<pre>
<syntaxhighlight lang="python">
sister_age = 15
sister_age = 15
brother_age = 12
brother_age = 12
Line 472: Line 485:
else:
else:
     print("brother is older")
     print("brother is older")
</pre>
</syntaxhighlight>


You don't have to have an <code>else</code> block, if you don't need it. That just means there isn't default code to execute when none of the <code>if</code> or <code>elif</code> conditions are <code>True</code>:
You don't have to have an <code>else</code> block, if you don't need it. That just means there isn't default code to execute when none of the <code>if</code> or <code>elif</code> conditions are <code>True</code>:


<pre>
<syntaxhighlight lang="python">
color = "orange"
color = "orange"
if color == "green" or color == "red":
if color == "green" or color == "red":
Line 484: Line 497:
elif color == "pink":
elif color == "pink":
   print("Valentine's Day color!")
   print("Valentine's Day color!")
</pre>
</syntaxhighlight>


If color had been "purple", that code wouldn't have printed anything.
If color had been "purple", that code wouldn't have printed anything.
Line 505: Line 518:


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 '''boolean''' that is helpful for writing code that makes decisions. There are two booleans: <code>True</code> and <code>False</code>.
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 '''boolean''' that is helpful for writing code that makes decisions. There are two booleans: <code>True</code> and <code>False</code>.
 
<syntaxhighlight lang="python">
  True
  True


Line 513: Line 526:


  type(False)
  type(False)
 
</syntaxhighlight>
You can test if Python objects are equal or unequal. The result is a boolean:
You can test if Python objects are equal or unequal. The result is a boolean:
 
<syntaxhighlight lang="python">
  0 == 0
  0 == 0


  0 == 1
  0 == 1
 
</syntaxhighlight>
Use <code>==</code> to test for equality. Recall that <code>=</code> is used for <i>assignment</i>.
Use <code>==</code> to test for equality. Recall that <code>=</code> is used for <i>assignment</i>.


Line 525: Line 538:


Use <code>!=</code> to test for inequality:
Use <code>!=</code> to test for inequality:
 
<syntaxhighlight lang="python">
  "a" != "a"
  "a" != "a"


  "a" != "A"
  "a" != "A"
 
</syntaxhighlight>
<code>&lt;</code>, <code>&lt;=</code>, <code>&gt;</code>, and <code>&gt;=</code> have the same meaning as in math class. The result of these tests is a boolean:
<code>&lt;</code>, <code>&lt;=</code>, <code>&gt;</code>, and <code>&gt;=</code> have the same meaning as in math class. The result of these tests is a boolean:
 
<syntaxhighlight lang="python">
  1 > 0
  1 > 0


Line 539: Line 552:


  .5 <= 1
  .5 <= 1
 
</syntaxhighlight>
You can check for containment with the <code>in</code> keyword, which also results in a boolean:
You can check for containment with the <code>in</code> keyword, which also results in a boolean:
 
<syntaxhighlight lang="python">
  "H" in "Hello"
  "H" in "Hello"


  "X" in "Hello"
  "X" in "Hello"
 
</syntaxhighlight>
Or check for a lack of containment with <code>not in</code>:
Or check for a lack of containment with <code>not in</code>:
 
<syntaxhighlight lang="python">
  "a" not in "abcde"
  "a" not in "abcde"


  "Perl" not in "Python Workshop"
  "Perl" not in "Python Workshop"
 
</syntaxhighlight>
==End of Part 2==
==End of Part 2==


Line 559: Line 572:


Take a break, stretch, meet some neighbors, and ask the staff if you have any questions about this material.
Take a break, stretch, meet some neighbors, and ask the staff if you have any questions about this material.
[[Category:Spring_2015_series]]

Latest revision as of 00:14, 17 September 2015

Welcome to the Friday tutorial!

This tutorial covers several core programming concepts that we'll build upon during an interactive lecture tomorrow morning. It will take 1-2 hours to complete. There's a break in the middle, and exercises at the middle and end to help review the material.

This is an interactive tutorial! As you go through this tutorial, any time you see something that looks like this:

 a = "Hello"

you should type the expression at a Python prompt, hitting Return after every line and noting the output.

No copying and pasting! You'll learn the concepts better if you type them out yourself.

Math[edit]

Calculator.png

Math in Python looks a lot like math you type into a calculator. A Python prompt makes a great calculator if you need to crunch some numbers and don't have a good calculator handy.

Addition[edit]

 2 + 2
 1.5 + 2.25

Subtraction[edit]

 4 - 2
 100 - .5
 0 - 2

Multiplication[edit]

 2 * 3

Division[edit]

First, lets try some basic fractions:

 1 / 2

Sure enough, Python says that the answer is 0.5 which is the same as 1/2.

Let's try the same thing:

 4 / 2

Hey now! That last result is a little strange. When you divide a number in Python, even if the answer doesn't need a decimal place (like 4/2 which equals 2), you get an answer with a decimal point!

What's going on here is that in Python, division produces an what's called a float which essentially means a number with a decimal point.

When the Python interpreter goes to do the division, it knows that (unlike multiplication for example) division can lead to numbers that aren't whole numbers (like 1/2) so it makes sure that the result will always include a decimal place just in case it's needed.


This shouldn't be important for this workshop but it might be worth knowing that older versions of Python (before version 3) would always round down and return integers instead of giving numbers with decimal places.

Types[edit]

Geometry.png

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

 type(1)
 type(1.0)

So now we've seen two data types: integers and floats.

By the way, what is a "function"? Here are the important ideas about functions:

  • A function encapsulates a useful bit of work and gives that work a name.
  • You provide input to a function and it produces output. For example, the type function takes data as an input, and produces what type of data the data is (e.g. an integer or a float) as output.
  • 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 arguments to the function), and then a close parenthesis.
  • Programmers have a lot of slang around functions. They'll say that functions "take" arguments, or that they "give" or "pass" arguments to a function. "call" and "invoke" are both synonyms for using a function.

In the example above, "type" was the name of the function. type takes one argument; we first gave type an argument of 1 and then gave it an argument of 1.0.

Diagram of "calling" a function[edit]

Function diagram.png

Command history[edit]

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

Variables[edit]

Fraction.png

A lot of work gets done in Python using variables. Variables are a lot like the variables in math class, except that in Python variables can be of any data type, not just numbers.

 type(4)
 x = 4
 x
 type(x)
 2 * x

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

Variables can't have spaces or other special characters, and they need to start with a letter. Here are some valid variable names:

 magic_number = 1500
 amountOfFlour = .75
 my_name = "Jessica"

Projects develop naming conventions: maybe multi-word variable names use underscores (like magic_number), or "camel case" (like amountOfFlour). The most important thing is to be consistent within a project, because it makes the code more readable.

Output[edit]

Pacman.png

Notice how if you type a 4 and hit enter, the Python interpreter spits a 4 back out:

 4

But if you assign 4 to a variable, nothing is printed:

 x = 4

You can think of it as that something needs to get the output. Without an assignment, the winner is the screen. With assignment, the output goes to the variable.

You can reassign variables if you want:

 x = 4
 x
 x = 5
 x

Sometimes reassigning a variable is an accident and causes bugs in programs.

 x = 3
 y = 4
 x * y
 x * x
 2 * x - 1 * y

Order of operations works pretty much like how you learned in school. If you're unsure of an ordering, you can add parentheses like on a calculator:

 (2 * x) - (1 * y)

Note that the spacing doesn't matter:

 x = 4

and

 x=4

are both valid Python and mean the same thing.

 (2 * x) - (1 * y)

and

 (2*x)-(1*y)

are also both valid and mean the same thing. You should strive to be consistent with whatever spacing you like or a job requires, since it makes reading the code easier.

You aren't cheating and skipping typing these exercises out, are you? Good! :)

Strings[edit]

Letter.png

So far we've seen two data types: integers and floats. Another useful data type is a string, 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:

 "Hello"
 "Python, I'm your #1 fan!"

Like with the math data types above, we can use the type function to check the type of strings:

 type("Hello")
 type(1)
 type("1")

String Concatenation[edit]

You can smoosh strings together (called "concatenation") using the '+' sign:

 "Hello" + "World"

 name = "Jessica"
 "Hello " + name

How about concatenating different data types?

 "Hello" + 1

Hey now! The output from the previous example was really different and interesting; let's break down exactly what happened:

>>> "Hello" + 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects

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

There are many kinds of Python errors, with descriptive names to help us humans understand what went wrong. In this case we are getting a TypeError: we tried to do some operation on a data type that isn't supported for that data type.

Python gives us a helpful error message as part of the TypeError:

"cannot concatenate 'str' and 'int' objects"

We saw above the we can concatenate strings:

"Hello" + "World"

works just fine.

However,

"Hello" + 1

produces a TypeError. We are telling Python to concatenate a string and an integer, and that's not something Python understands how to do.

We can convert an integer into a string ourselves, using the str function:

"Hello" + str(1)

Like the type function from before, the str function takes 1 argument. In the above example it took the integer 1. str takes a Python object as input and produces a string version of that input as output.

String length[edit]

There's another useful function that works on strings called len. len returns the length of a string as an integer:

 len("Hello")
 len("")
 fish = "humuhumunukunukuapua'a"
 name_length = len(fish)
 fish + " is a Hawaiian fish whose name is " + str(name_length) + " characters long."

Quotes[edit]

We've been using double quotes around our strings, but you can use either double or single quotes:

 'Hello'
 "Hello"

Like with spacing above, use whichever quotes make the most sense for you, but be consistent.

You do have to be careful about using quotes inside of strings:

'I'm a happy camper'

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

m a happy camper'

which it doesn't understand -- it's not 'valid' Python. Those letters aren't variables (we haven't assigned them to anything), and that trailing quote isn't balanced. So it raises a SyntaxError.

We can use double quotes to avoid this problem:

"I'm a happy camper"

One fun thing about strings in Python is that you can multiply them:

 "A" * 40
 "ABC" * 12
 h = "Happy"
 b = "Birthday"
 (h + b) * 10

Part 1 Practice[edit]

Detective.png

Read the following expressions, but don't execute them. Guess what the output will be. After you've made a guess, copy and paste the expressions at a Python prompt and check your guess.

1.

 total = 1.5 - 1/2
 total
 type(total)

2.

 a = "quick"
 b =  "brown"
 c = "fox jumps over the lazy dog"
 "The " +  a * 3 + " " +  b * 3 + " " + c

End of Part 1[edit]

Congratulations! You've learned about and practiced math, strings, variables, data types, exceptions, tracebacks, and executing Python from the Python prompt.

Take a break, stretch, meet some neighbors, and ask the staff if you have any questions about this material.

Fireworks.png

Part 2: Printing[edit]

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

 h = "Hello"
 w = "World"
 h + w

will display "HelloWorld".

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 print function. Here's how it works:

 h = "Hello"
 w = "World"
 print(h + w)

 my_string = "Alpha " + "Beta " + "Gamma " + "Delta"
 print(my_string)

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

   h + w

becomes

   print(h + w)

We'll see more examples of the print function in the next section.

Python scripts[edit]

Treasure map.png

Until now we've been using the interactive Python interpreter. This is great for learning and experimenting, but you can't easily save or edit your work. For bigger projects, we'll write our Python code in a file. Let's get some practice with this!

  1. Download the file http://mako.cc/teaching/2015/cdsw-spring/nobel.py by right-clicking on it and saying to save it as a ".py" file to your Desktop. The ".py" extension hints that this is a Python script.
  2. Open a terminal prompt, and use the navigation commands (dir and cd on Windows, ls, pwd, and cd on OS X and Linux) to navigate to your Desktop directory. See navigating from a terminal for a refresher on those commands.
  3. Once you are in your Desktop directory, execute the contents of nobel.py by typing python nobel.py at the terminal prompt. nobel.py introduces two new concepts: comments and multiline strings.
  4. Open nobel.py in your text editor (see preparing your text editor for a refresher on starting the editor).
  5. Read through the file in your text editor carefully and check your understanding of both the comments and the code.

Study the script until you can answer these questions:

  1. How do you comment code in Python?
  2. How do you print just a newline?
  3. How do you print a multi-line string so that whitespace is preserved?

Making choices[edit]

We can use these expressions that evaluate to booleans to make decisions and conditionally execute code.

Fork.png

if statements[edit]

The simplest way to make a choice in Python is with the if keyword. Here's an example (don't try to type this one, just look at it for now):

    if 6 > 5:
        print("Six is greater than five!")

That is our first multi-line piece of code, and the way to type it at a Python prompt is a little different. Let's break down how to do this (type this out step by step):

  1. First, type the

        if 6 > 5:

    part, and press Enter. The next line will have ... as a prompt, instead of the usual >>>. This is Python telling us that we are in the middle of a code block, and so long as we indent our code it should be a part of this code block.
  2. Press the spacebar 4 times to indent.
  3. Type

        print("Six is greater than five!")

  4. Press Enter to end the line. The prompt will still be a ...
  5. Press Enter one more time to tell Python you are done with this code block. The code block will now execute.

All together, it will look like this:

>>> if 6 > 5:
...      print("Six is greater than five!")
... 
Six is greater than five!

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

In this case, because 6 really is greater than 5, Python executes the code block under the if statement, and we see "Six is greater than five!" printed to the screen. Guess what will happen with these other expressions, then type them out and see if your guess was correct:

if 0 > 2:
     print("Zero is greater than two!")
if "banana" in "bananarama":
    print("I miss the 80s.")

more choices: if and else[edit]

if lets you execute some code only if a condition is True. What if you want to execute different code if a condition is False?

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

sister_age = 15
brother_age = 12
if sister_age > brother_age:
    print("sister is older")
else:
    print("brother is older")

Like with if, the code block under the else condition must be indented so Python knows that it is a part of the else block.

compound conditionals: and and or[edit]

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

Try typing these out and see what you get:

1 > 0 and 1 < 2
1 < 2 and "x" in "abc"
"a" in "hello" or "e" in "hello"
1 <= 0 or "a" not in "abc"

Guess what will happen when you enter these next two examples, and then type them out and see if you are correct. If you have trouble with the indenting, call over a staff member and practice together. It is important to be comfortable with indenting for tomorrow.

temperature = 32
if temperature > 60 and temperature < 75:
    print("It's nice and cozy in here!")
else:
    print("Too extreme for me.")
hour = 11
if hour < 7 or hour > 23:
    print("Go away!")
    print("I'm sleeping!")
else:
    print("Welcome to the cheese shop!")
    print("Can I interest you in some choice gouda?")

You can have as many lines of code as you want in if and else blocks; just make sure to indent them so Python knows they are a part of the block.

even more choices: elif[edit]

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

sister_age = 15
brother_age = 12
if sister_age > brother_age:
    print("sister is older")
elif sister_age == brother_age:
    print("sister and brother are the same age")
else:
    print("brother is older")

You don't have to have an else block, if you don't need it. That just means there isn't default code to execute when none of the if or elif conditions are True:

color = "orange"
if color == "green" or color == "red":
  print("Christmas color!")
elif color == "black" or color == "orange":
  print("Halloween color!")
elif color == "pink":
  print("Valentine's Day color!")

If color had been "purple", that code wouldn't have printed anything.

Remember that '=' is for assignment and '==' is for comparison.

In summary: the structure of if/elif/else[edit]

Here's a diagram of if/elif/else:

If-elif-else.png

Do you understand the difference between elif and else? When do you indent? When do you use a colon? If you're not sure, talk about it with a neighbor or staff member.

Booleans[edit]

Please return to the interactive Python interpreter for the rest of the tutorial. And remember: type out the examples. You'll thank yourself tomorrow. :)

Scales.png

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

 True

 type(True)

 False

 type(False)

You can test if Python objects are equal or unequal. The result is a boolean:

 0 == 0

 0 == 1

Use == to test for equality. Recall that = is used for assignment.

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

Use != to test for inequality:

 "a" != "a"

 "a" != "A"

<, <=, >, and >= have the same meaning as in math class. The result of these tests is a boolean:

 1 > 0

 2 >= 3

 -1 < 0

 .5 <= 1

You can check for containment with the in keyword, which also results in a boolean:

 "H" in "Hello"

 "X" in "Hello"

Or check for a lack of containment with not in:

 "a" not in "abcde"

 "Perl" not in "Python Workshop"

End of Part 2[edit]

Congratulations! You've learned about and practiced executing Python scripts, booleans, conditionals, and making choices with if, elif, and else. This is a huge, huge accomplishment!

Champagne.pngParty.png

Take a break, stretch, meet some neighbors, and ask the staff if you have any questions about this material.