Montreal Python Workshop/Friday/Tutorial
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 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.
2 + 2 1.5 + 2.25
4 - 2 100 - .5 0 - 2
2 * 3
4 / 2 1 / 2
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
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:
3/4 + 1/4
What do you need to do to get the right answer? Use data types that understand decimals for each of the divisions:
3.0/4 + 1.0/4 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".
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:
So now we've seen two data types: integers and floats.
I used the term 'function' without explaining what it is -- we'll talk about functions more in a bit, and write our own, but for now know these things:
- Functions encapsulate some useful bit of work. We save that useful bit of work inside the function so we don't have to type it over and over again every time we want to use it. So, for example, some nice person decided that being able to determine the type of an object was useful, so he or she put the Python code that figures out an object's type into the function
type, and now we all get to use it, instead of having to write it ourselves.
- Functions are sort of like functions in math class. You provide input to a function and it produces output. The
typefunction 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, what the function takes as input (we call that input the arguments to the function), and then a close parenthesis.
So in this case 'type' is the name of the function, and it takes one argument; in the example we first give
type an argument of 1 and then give it an argument of 1.0.
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!
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: multi-word variable names most often 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. The recommended style is with_underscore.
Notice how if you type a 4 and hit enter, the Python interpreter spits a 4 back out:
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
are both valid Python and mean the same thing. The first version is a little easier to read for people and is thus recommended.
(2 * x) - (1 * y)
are also both valid and mean the same thing.
You aren't cheating and skipping typing these exercises out, are you? Good! :)
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")
You can smoosh strings together (called "concatenation") using the '+' sign:
"Hello" + "World"
name = "Jessica" print "Hello " + name
You can print strings using
h = "Hello" w = "World" print h + w
my_string = "Alpha " + "Beta " + "Gamma " + "Delta" print my_string
How about printing different data types together?
print "Hello" + 1
Hey now! The output from the previous example was really different and interesting; let's break down exactly what happened:
>>> print "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:
print "Hello" + "World"
works just fine.
print "Hello" + 1
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
print "Hello" + str(1)
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.
There's another useful function that works on strings called
len returns the length of a string as an integer:
print len("Hello") print len("") fish = "humuhumunukunukuapuaʻa" length = str(len(fish)) print fish + " is a Hawaiian fish whose name is " + length + " characters long."
We've been using double quotes around our strings, but you can use either double or single quotes:
print 'Hello' print "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:
print '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
We can use double quotes to avoid this problem:
print "I'm a happy camper"
One fun thing about strings in Python is that you can multiply them:
print "A" * 40 print "ABC" * 12 h = "Happy" b = "Birthday" print (h + b) * 10
Part 1 Practice
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.
total = 1.5 - 1/2 print total type(total)
a = "quick" b = "brown" c = "fox jumps over the lazy dog" print "The " + a * 3 + " " + b * 3 + " " + c
End of Part 1
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.
Until now we've been executing commands at the Python prompt. This is great for math, short bits of code, and testing. For longer ideas, it's easier to store the code in a file.
- Download the file http://mit.edu/jesstess/www/BostonPythonWorkshop7/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.
- Open a command prompt, and use the navigation commands (
cdon OS X and Linux) to navigate to your home directory. See the pages about Setup for a refresher on those commands (ask the staff if you forgot the link).
- Once you are in your home directory, execute the contents of
at a command prompt.
nobel.pyintroduces two new concepts: comments and multiline strings.
nobel.pyin your text editor (see preparing your text editor for a refresher on starting the editor).
- 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:
- How do you comment code in Python?
- How do you print just a newline?
- How do you print a multi-line string so that whitespace is preserved?
Let's get back to some interactive examples. Keep typing them out! You'll thank yourself tomorrow. :)
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:
You can test if Python objects are equal or unequal. The result is a boolean:
0 == 0
0 == 1
== 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.
!= to test for inequality:
"a" != "a"
"a" != "A"
>= 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
"a" not in "abcde"
"Perl" not in "Boston Python Workshop"
We can use these expressions that evaluate to booleans to make decisions and conditionally execute code:
if 6 > 5: print "Six is greater than five!"
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
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.
Enter 4 spaces, and then type
print "Six is greater than five!"
Press Enter to end the line, and press Enter again to tell Python you are done with this code block. 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."
You can use the
else keyword to execute code when the
if expression 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"
if, the code block under the
else statement must be indented so Python knows that it is a part of the
You can check multiple expressions together using the
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 = -17 if temperature > 18 and temperature < 28: 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
else blocks; just make sure to indent them so Python knows they are a part of the block.
even more choices:
If you have 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
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
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
Here's a diagram of
Do you understand the difference between
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.
We talked a bit about functions when we introduced the
type() function. Let's review what we know about functions:
- They do some useful bit of work.
- They let us re-use code without having to type it out each time.
- They take input and possibly produce output (we say they return a value). You can assign a variable to this output.
- You call a function by using its name followed by its arguments in parenthesis.
length = len("Mississippi")
Executing this code assigns the length of the string "Mississippi" to the variable
We can write our own functions to encapsulate bits of useful work so we can reuse them. Here's how you do it:
Step 1: write a function signature
A function signature tells you how the function will be called. It starts with the keyword
def, which tells Python that you are defining a function. Then comes a space, the name of your function, an open parenthesis, the comma-separated input arguments for your function, a close parenthesis, and a colon. Here's what a function signature looks like for a function that takes no arguments:
Here's what a function signature looks like for a function that takes one argument called
And one for a function that takes two arguments:
def greet(name, title):
Arguments should have names that usefully describe what they are used for in the function.
Step 2: do useful work inside the function
Underneath the function signature you do your useful work. Everything inside the function is indented, just like with if/else blocks, so Python knows that it is a part of the function.
You can use the variables passed into the function as arguments, just like you can use variables once you define them outside of functions.
def add(x, y): result = x + y
Step 3: return something
If you want to be able to assign a variable to the output of a function, the function has to return that output using the
def add(x, y): result = x + y return result
or, even shorter:
def add(x, y): return x + y
You can return any Python object: numbers, strings, booleans ... even other functions!
Once you execute a return, you are done with the function -- you don't get to do any more work. That means if you have a function like this:
def get_absolute_value(number): if number < 0: return number * -1 return number
number is less than 0, you return
number * -1 and never even get to the last line of the function. However, if
number is greater than or equal to 0, the
if expression evaluates to
False, so we skip the code in the
if block and return
We could have written the above function like this if we wanted. It's the same logic, just more typing:
def get_absolute_value(number): if number < 0: return number * -1 else: return number
Step 4: use the function
Once you define a function you can use it as many times as you want:
def add(x, y): return x + y result = add(1234, 5678) print result result = add(-1.5, .5) print result
Functions don't have to return anything, if you don't want them to. They usually return something because we usually want to be able to assign variables to their output.
What is the difference between
Think for a moment about the differences between
returnis used to hand off a value from inside a function to a variable outside the function.
def add(x, y): print x + y
x + y to the screen so your eyes can see it.
def add(x, y): return x + y
will hand off
x + y from inside the function to outside the function. This allows you to do something like:
result = add(5, 6) print result
Does that make sense? If not, talk about it with a neighbor or staff member.
End of Part 2
Congratulations! You've learned about and practiced executing Python scripts, booleans, conditionals, and if/else blocks, and you've written your own Python functions. This is a huge, huge accomplishment!
Take a break, stretch, meet some neighbors, and ask the staff if you have any questions about this material.