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.
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
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, 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
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: 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.
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.
(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! :)
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" "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.
"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
"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:
len("Hello") len("") fish = "humuhumunukunukuapuaʻa" name_length = len(fish) fish + " is a Hawaiian fish whose name is " + str(name_length) + " characters long."
We've been using double quotes around our strings, but you can use either double or single quotes:
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
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 2: Printing
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
print(h + w)
We'll see more examples of the print function in the next section.
Please return to the interactive Python interpreter for the rest of the tutorial. And remember: type out the examples. 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.
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):
- 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.
- Press the spacebar 4 times to indent.
print("Six is greater than five!")
- Press Enter to end the line. The prompt will still be a
- 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.")
if lets you execute some code only if a condition is
True. What if you want to execute different code if a condition is
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")
if, the code block under the
else condition 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
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
else blocks; just make sure to indent them so Python knows they are a part of the block.
even more choices:
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
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
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.