So how do you fix them?

Let's start with some common errors!

def first: # SyntaxError

None = 1 # SyntaxError

return # SyntaxError

This occurs when a variable is not defined, i.e. it hasn't been assigned

test
# NameError: name 'test' is not defined

len(5)  
# TypeError: object of type 'int' has no len()

"awesome" + []  
# TypeError: cannot concatenate 'str' and 'list' objects

​Occurs when:

Occurs when you try to access an element in a list using an invalid index (i.e. one that is outside the range of the list or string):

list = ["hello"]
list[2]
# IndexError: list index out of range

This occurs when a built-in operation or function receives an argument that has the right type but an inappropriate value:

int("foo")
# ValueError: invalid literal for int() with base 10: 'foo'

This occurs when a dictionary does not have a specific key:

d = {}
d["foo"]
# KeyError: 'foo'

This occurs when a variable does not have an attribute:

"awesome".foo
# AttributeError: 'str' object has no attribute 'foo'

In python we can also throw errors using the raise keyword. This is helpful when creating your own kinds of exception and error messages.

raise ValueError('invalid value')

In Python, it is strongly encouraged to use try/except blocks, to catch exceptions when we can do something about them. Let's see what that looks like.

try: 
    foobar
except NameError as err:
    print(err)

What we are doing here is catching every error, which means we are not able to correctly identify "what" went wrong. It is highly discouraged to do this. 

try: 
    colt
except:
    print("You tried to use a variable that was never declared!")

When you use try/except, make sure that a specific type of exception is being handled.

try: 
    colt
except NameError:
    print("You tried to use a variable that was never declared!")

​If you want to except a handful of exceptions, you can pass a tuple of errors into the except block as well:

try: 
    colt.hello
except (TypeError, AttributeError):
    print("That doesn't work with this thing.")

To set breakpoints in our code we can use pdb by inserting this line:

import pdb; pdb.set_trace() 

This is how we include modules! We'll learn about these in quite a bit more depth later. 

import pdb; pdb.set_trace() 

For now, think of this as loading some external code and then running code right away

Inside of the debugger we can press c to continue and q to quit.

def add_then_multiply(num1, num2):
    sum = num1 + num2
    import pdb; pdb.set_trace()

    product = sum * num1 * num2

    return product

There are a few more shortcuts as well; you can read more about pdb here.

Be careful with how you name your parameters!

def add_numbers(a, b, c, d):
    import pdb; pdb.set_trace() 

    return a + b + c + d