Boston Python workshop 2/Friday tutorial

Type each of the following expressions at the Python prompt (no copying and pasting! You'll learn the concepts better if you type them out yourself). After you type an expression, hit Return. This causes the Python interpreter to evaluate the expression.

Math

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

2 + 2
1.5 + 2.25

Subtraction

4 - 2
100 - .5
0 - 2

Multiplication

2 * 3

Division

4 / 2
1 / 2

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:

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.

Types

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.

I used the term 'function' without explaining what it is -- we'll talk about functions a lot more on Saturday, and write our own, but for now know two things:

• To use a function, write the name of the function followed by an open parenthesis, possibly some data (we call that data 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.

• Functions are a lot like functions in math class. You provide input to a function, and it possibly produces output. 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.

Command history

Stop here and try hitting the Up arrow on your keyboard. 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

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
divisible_by_6 = 54

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

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 syntax.

(2 * x) - (1 * y)

and

(2*x)-(1*y)

are also valid syntax. You should strive to be consistent with whatever syntax you like or a job requires, since it makes reading the code easier.

Strings

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:

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

String Concatenation

print "Hello" + "World"
print "Hello", "World"

h = "Hello"
w = "World"
print h + w
print h, w

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

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

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

Printing different data types together

print "Hello", 1
print "Hello" + 1

The output from the previous example is really different and interesting; let's break down exactly what is happening:

>>> 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.

However,

print "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.

In the similar expression

print "Hello", 1

Python is actually turning the integer 1 into a string before printing, and that's why the concatenation works: Python does know how to concatenate two strings.

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

print "Hello" + str(1)

Like the type function from before, the str function takes 1 argument, in the example the integer 1. str takes in input and produces a string version of that input as output.

Quotes

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 SyntaxError.

We can use double quotes to avoid this problem:

print "I'm a happy camper"

or we can escape the quote with a backslash:

print 'Ada Lovelace is often called the world\'s first programmer.'
print "Computer scientist Grace Hopper popularized the term \"debugging\"."

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.

1.

total = 1.5 - 1/2 + ((-2.0/2) - (1.0/2))
print total
type(total)

2.

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

3.

print 2.0 * 123, "2.0" * 123
print 2.0 * 123 + str(2.0) * 123

4.

a = "| (_|   -()-  -()-   -()-   -()- | -()-  -()-  -()-   -()-   ||\n"
b = "|_\_|_/___|__|__|__|___|__|___|__|___________________________||\n"
c = "|________________________________|__|__()_|__()_|__()__|_____||\n"
d = " ___|)_______________________________________________________\n"
e = "|_/(|,\____/_|___/_|____/_|______|___________________________||\n"
f = "|___/____________________________|___________________________||\n"
g = "|   |     | ()  | ()   | ()   |  |                           ||\n"
h = "|__\___|.________________________|___\_|___\_|___\_|___|_____||\n"
i = "|__/|_______/|____/|_____/|______|___________________________||\n"
j = "|_____/__________________________|____\|____\|____\|_________||\n"
k = "|____/___________________________|___________________________||\n"
l = "|__/___\_._______________________|__|__|__|__|__|__|___|_____||\n"

print d + f + i + e + b + g + a + c + l + h + j + k

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 and from a file.

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

Python scripts

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.

1. Download the file http://mit.edu/jesstess/www/BostonPythonWorkshop2/nobel.py. The ".py" extension hints that this is a Python script.
2. Save the file in your Desktop directory.
3. Open a command prompt, and use the navigation commands (dir and cd on Windows, ls, pwd, and cd on OS X and Linux) to navigate to your home directory. See navigating from a command prompt for a refresher on those commands.
4. Once you are in your home directory, execute the contents of nobel.py by typing

   python nobel.py
   

   at a command prompt.

   nobel.py introduces two new concepts: comments and multiline strings.
5. Open nobel.py in your text editor (see preparing your text editor for a refresher on starting the editor).
6. Read through the file in your text editor carefully and check your understanding of both the comments and the code.

Booleans

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:

0 == 0

0 == 1

Use == to test for equality. Recall that = is used for assignment (e.g. my_string == "Hello").

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:

1 > 0

2 >= 3

-1 < 0

.5 <= 1

You can check for containment with the in keyword:

"H" in "Hello"

"X" in "Hello"

Or check for a lack of containment with not in:

"a" not in "abcde"

"Elvis" not in "Elvis has left the building"

Flow Control

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

if True:
     print "I'm True!"

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 True: part, and hit 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.

Type 4 spaces, and then type print "I'm True!". 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:

>>> if True:
...     print "I'm True!"
... 
"I'm True!"

So 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 1 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 with this one:

if False:
     print "I'm True!"

if 1:
    print "1 is truthy"

if 0:
    print "0 is truthy"

if "test":
    print "non-empty strings are truthy"

if "":
    print "empty strings are truthy"

You can use the else keyword to conditionally execute code when the expression for the if block isn't true:

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 statement must be indented.

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 one is True, the overall expression is True.

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"

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?"

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