Python 2 hour intro

This material is intended for a 1.5 - 2 hour interactive lecture.

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

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".

Command history

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

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

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

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

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

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

"Hello" + "World"

name = "Jessica"
print "Hello " + name

Printing

You can print strings using print:

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.

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.

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 above example it took the integer 1. str takes a Python object as 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 'I\'m a happy camper'
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

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 = 3 - 1/2 + .5
print total

2.

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.

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.

Download the file http://mit.edu/jesstess/www/BostonPythonWorkshop6/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.
Run the script by double-clicking on it. What happens?

nobel.py

Open nobel.py in a text 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.

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. 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 "Boston Python Workshop"

Flow Control

if statements

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 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 "Six is greater than five!". 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 like this:

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

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 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`

You can use the else keyword to execute code only when the if expression 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 so Python knows that it is a part of the else block.

compound conditionals: `and` and `or`

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`

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 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.

Lists

Use lists to store data where order matters.
Lists are indexed starting with 0.

List initialization

>>> my_list = []
>>> my_list
[]
>>> your_list = ["a", "b", "c", 1, 2, 3]
>>> your_list
['a', 'b', 'c', 1, 2, 3]

Access and adding elements to a list

>>> len(my_list)
0
>>> my_list[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range
>>> my_list.append("Alice")
>>> my_list
['Alice']
>>> len(my_list)
1
>>> my_list[0]
'Alice'
>>> my_list.insert(0, "Amy")
>>> my_list
['Amy', 'Alice']

>>> my_list = ['Amy', 'Alice']
>>> 'Amy' in my_list
True
>>> 'Bob' in my_list
False

Changing elements in a list

>>> your_list = []
>>> your_list.append("apples")
>>> your_list[0]
'apples'
>>> your_list[0] = "bananas"
>>> your_list
['bananas']

Slicing lists

>>> her_list = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
>>> her_list[0]
'a'
>>> her_list[0:3]
['a', 'b', 'c']
>>> her_list[:3]
['a', 'b', 'c']
>>> her_list[-1]
'h'
>>> her_list[5:]
['f', 'g', 'h']
>>> her_list[:]
['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']

Strings are a lot like lists

>>> my_string = "Hello World"
>>> my_string[0]
'H'
>>> my_string[:5]
'Hello'
>>> my_string[6:]
'World'
>>> my_string = my_string[:6] + "Jessica"
>>> my_string
'Hello Jessica'

One big way in which strings are different from lists is that lists are mutable (you can change them), and strings are immutable (you can't change them). To "change" a string you have to make a copy:

>>> h = "Hello"
>>> h[0] = "J"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> h = "J" + h[1:]
>>> h
'Jello'

Loops

Use a for loop to do something to every element in a list.

>>> names = ["Jessica", "Adam", "Liz"]
>>> for name in names:
...     print name
...
Jessica
Adam
Liz

>>> names = ["Jessica", "Adam", "Liz"]
>>> for name in names:
...     print "Hello " + name
...
Hello Jessica
Hello Adam
Hello Liz

End of Part 2

Congratulations! You've learned about and practiced writing and executing Python scripts, making choices with booleans and conditionals, and using lists and iteration. This is a huge, huge accomplishment!