Extra - Errors and Exceptions

Overview

Teaching: 30 min
Exercises: 0 min

Questions

• How does Python report errors?

• How can I handle errors in Python programs?

Objectives

• To be able to read a traceback, and determine where the error took place and what type it is.

• To be able to describe the types of situations in which syntax errors, indentation errors, name errors, index errors, and missing file errors occur.

Handling Errors

How Things Go Wrong: Types of Errors

At this point, you have surely encountered a variety of errors. Errors are Python’s way of telling you that it can’t do what you think you’ve requested. This could be because you are talking about data it can’t find, writing logic it can’t follow, or trying to run code that isn’t well-formed.

The first step in diagnosing what has gone wrong is to know how to read an error report. This is called a traceback. Let’s examine one:

# This code has an intentional error. You can type it directly or
# use it for reference to understand the error message below.
def favorite_ice_cream():
    ice_creams = [
        "chocolate",
        "vanilla",
        "strawberry"
    ]
    print(ice_creams[3])

favorite_ice_cream()

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-1-70bd89baa4df> in <module>()
      6     print(ice_creams[3])
      7
----> 8 favorite_ice_cream()

<ipython-input-1-70bd89baa4df> in favorite_ice_cream()
      4         "vanilla",                                                                    "strawberry"
      5     ]
----> 6     print(ice_creams[3])
      7
      8 favorite_ice_cream()

IndexError: list index out of range

This particular traceback has two levels, but you may find tracebacks with only a single layer or with close to a dozen layers. (You can determine the number of levels by looking for the number of arrows on the left hand side.) In this case:

1. The first shows code from the cell above, with an arrow pointing to Line 8 (which is favorite_ice_cream()).

2. The second shows some code in the function favorite_ice_cream, with an arrow pointing to Line 6 (which is print(ice_creams[3])).

The last level is the actual place where the error occurred. The other level(s) show what function the program executed to get to that level. Most of the time, we only really care about the bottommost message (or innermost layer).

In this case, the program first performed a function call to the function favorite_ice_cream. Inside this function, the program encountered an error on Line 6, when it tried to run the code print(ice_creams[3]).

In the last line of the traceback, Python helpfully tells us the category or type of error (in this case, it is an IndexError) and a more detailed error message (in this case, it says “list index out of range”).

Long Tracebacks

Sometimes, you might see a traceback that is very long – sometimes they might even be twenty levels deep! This can make it seem like something horrible happened, but really it just means that your program called many functions before it ran into the error.

Most of the time, you can just pay attention to the bottom-most level, which is the actual place where the error occurred.

Once we know where something went wrong, our next step is to figure out why. The type of error can be quite helpful in diagnosing and correcting the error.

What are some common exceptions you may have seen up to this point?

• NameError
• SyntaxError
• IndexError and KeyError
• FileNotFoundError and relatives

If you encounter an error and don’t know what it means, it is still important to read the traceback closely. That way, if you fix the error, but encounter a new one, you can tell that the error changed. Additionally, sometimes just knowing where the error occurred is enough to fix it, even if you don’t entirely understand the message.

If you do encounter an error you don’t recognize, try looking at the official documentation on errors.

Challenge: Reading Error Messages

Read the python code and the resulting traceback below, and answer the following questions:

1. How many levels does the traceback have?
2. What is the function name where the error occurred?
3. On which line number in this function did the error occurr?
4. What is the type of error?
5. What is the error message?

# This code has an intentional error. Do not type it directly;
# use it for reference to understand the error message below.
def print_message(day):
    messages = {
        "monday": "Hello, world!",
        "tuesday": "Today is tuesday!",
        "wednesday": "It is the middle of the week.",
        "thursday": "Today is Donnerstag in German!",
        "friday": "Last day of the week!",
        "saturday": "Hooray for the weekend!",
        "sunday": "Aw, the weekend is almost over."
    }
    print(messages[day])

def print_friday_message():
    print_message("Friday")

print_friday_message()

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-1-4be1945adbe2> in <module>()
     14     print_message("Friday")
     15
---> 16 print_friday_message()

<ipython-input-1-4be1945adbe2> in print_friday_message()
     12
     13 def print_friday_message():
---> 14     print_message("Friday")
     15
     16 print_friday_message()

<ipython-input-1-4be1945adbe2> in print_message(day)
      9         "sunday": "Aw, the weekend is almost over."
     10     }
---> 11     print(messages[day])
     12
     13 def print_friday_message():

KeyError: 'Friday'

Solution

1. 3 levels
2. print_message
3. 11
4. KeyError
5. There isn’t really a message; you’re supposed to infer that Friday is not a key in messages.

Syntax Errors

When you forget a colon at the end of a line, accidentally add one space too many when indenting under an if statement, or forget a parenthesis, you will encounter a syntax error. This means that Python couldn’t figure out how to read your program. This is similar to forgetting punctuation in English: for example, this text is difficult to read there is no punctuation there is also no capitalization why is this hard because you have to figure out where each sentence ends you also have to figure out where each sentence begins to some extent it might be ambiguous if there should be a sentence break or not

People can typically figure out what is meant by text with no punctuation, but people are much smarter at inferring context than computers. If Python doesn’t know how to read the program, it will just give up and inform you with an error. For example:

def some_function()
    msg = "hello, world!"
    print(msg)
     return msg

  File "<ipython-input-3-6bb841ea1423>", line 1
    def some_function()
                       ^
SyntaxError: invalid syntax

Here, Python tells us that there is a SyntaxError on line 1, and even puts a little arrow in the place where there is an issue. In this case the problem is that the function definition is missing a colon at the end.

Actually, the function above has two issues with syntax. If we fix the problem with the colon, we see that there is also an IndentationError, which means that the lines in the function definition do not all have the same indentation:

def some_function():
    msg = "hello, world!"
    print(msg)
     return msg

  File "<ipython-input-4-ae290e7659cb>", line 4
    return msg
    ^
IndentationError: unexpected indent

Both SyntaxError and IndentationError indicate a problem with the syntax of your program, but an IndentationError is more specific: it always means that there is a problem with how your code is indented.

Tabs and Spaces

Some indentation errors are harder to spot than others. In particular, mixing spaces and tabs can be difficult to spot because they are both whitespace. The Jupyter notebook actually gives us a bit of a hint, but not all Python editors will do that. In the following example, the first two lines are using a tab for indentation, while the third line uses eight spaces:

def some_function():
	msg = "hello, world!"
	print(msg)
        return msg

Visually it is impossible to spot the error. Fortunately, Python does not allow you to mix tabs and spaces.

  File "<ipython-input-5-653b36fbcd41>", line 4
    return msg
              ^
TabError: inconsistent use of tabs and spaces in indentation

Challenge: Identifying Syntax Errors

1. Read the code below, and (without running it) try to identify what the errors are.
2. Run the code, and read the error message. Is it a SyntaxError or an IndentationError?
3. Fix the error.
4. Repeat steps 2 and 3, until you have fixed all the errors.

def another_function
  print("Syntax errors are annoying.")
   print("But at least python tells us about them!")
  print("So they are usually not too hard to fix.")

Solution

SyntaxError for missing (): at end of first line, IndentationError for mismatch between second and third lines. A fixed version is:

def another_function():
    print("Syntax errors are annoying.")
    print("But at least python tells us about them!")
    print("So they are usually not too hard to fix.")

Variable Name Errors

Another very common type of error is called a NameError, and occurs when you try to use a variable that does not exist. For example:

print(a)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-7-9d7b17ad5387> in <module>()
----> 1 print(a)

NameError: name 'a' is not defined

Variable name errors come with some of the most informative error messages, which are usually of the form “name ‘the_variable_name’ is not defined”.

Why does this error message occur? That’s a harder question to answer, because it depends on what your code is supposed to do. However, there are a few very common reasons why you might have an undefined variable. The first is that you meant to use a string, but forgot to put quotes around it:

print(hello)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-8-9553ee03b645> in <module>()
----> 1 print(hello)

NameError: name 'hello' is not defined

The second is that you just forgot to create the variable before using it. In the following example, count should have been defined (e.g., with count = 0) before the for loop:

for number in range(10):
    count = count + number
print("The count is:", count)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-9-dd6a12d7ca5c> in <module>()
      1 for number in range(10):
----> 2     count = count + number
      3 print("The count is:", count)

NameError: name 'count' is not defined

Finally, the third possibility is that you made a spelling error when you were writing your code. Let’s say we fixed the error above by adding the line Count = 0 before the for loop. Frustratingly, this actually does not fix the error. Remember that variables are case-sensitive, so the variable count is different from Count. We still get the same error, because we still have not defined count:

Count = 0
for number in range(10):
    count = count + number
print("The count is:", count)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-10-d77d40059aea> in <module>()
      1 Count = 0
      2 for number in range(10):
----> 3     count = count + number
      4 print("The count is:", count)

NameError: name 'count' is not defined

Challenge: Identifying Variable Name Errors

1. Read the code below, and (without running it) try to identify what the errors are.
2. Run the code, and read the error message. What type of NameError do you think this is? In other words, is it a string with no quotes, a misspelled variable, or a variable that should have been defined but was not?
3. Fix the error.
4. Repeat steps 2 and 3, until you have fixed all the errors.

for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (Number % 3) == 0:
        message = message + a
    else:
        message = message + "b"
print(message)

Solution

3 NameErrors for number being misspelled, for message not defined, and for a not being in quotes.

Fixed version:

message = ""
for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (number % 3) == 0:
        message = message + "a"
    else:
        message = message + "b"
print(message)

Index Errors

Next up are errors having to do with containers (like lists and strings) and the items within them. If you try to access an item in a list or a string that does not exist, then you will get an error. This makes sense: if you asked someone what day they would like to get coffee, and they answered “caturday”, you might be a bit surprised but not fundamentally confused. Python gets downright annoyed if you try to ask it for an item that doesn’t exist:

letters = ['a', 'b', 'c']
print("Letter #1 is", letters[0])
print("Letter #2 is", letters[1])
print("Letter #3 is", letters[2])
print("Letter #4 is", letters[3])

Letter #1 is a
Letter #2 is b
Letter #3 is c

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-11-d817f55b7d6c> in <module>()
      3 print("Letter #2 is", letters[1])
      4 print("Letter #3 is", letters[2])
----> 5 print("Letter #4 is", letters[3])

IndexError: list index out of range

Here, Python is telling us that there is an IndexError in our code, meaning we tried to access a list index that did not exist.

Challenge: Identifying Index Errors

1. Read the code below, and (without running it) try to identify what the errors are.
2. Run the code, and read the error message. What type of error is it?
3. Fix the error.

seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print('My favorite season is ', seasons[4])

Solution

IndexError; the last entry is seasons[3], so seasons[4] doesn’t make sense. A fixed version is:

seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print('My favorite season is ', seasons[-1])

File Errors

The last type of error we’ll cover today are those associated with reading and writing files: FileNotFoundError. If you try to read a file that does not exist, you will receive a FileNotFoundError telling you so. If you attempt to write to a file that was opened read-only, Python 3 returns an UnsupportedOperationError. More generally, problems with input and output manifest as IOErrors or OSErrors, depending on the version of Python you use.

file_handle = open('myfile.txt', 'r')

---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-14-f6e1ac4aee96> in <module>()
----> 1 file_handle = open('myfile.txt', 'r')

FileNotFoundError: [Errno 2] No such file or directory: 'myfile.txt'

One reason for receiving this error is that you specified an incorrect path to the file. For example, if I am currently in a folder called myproject, and I have a file in myproject/writing/myfile.txt, but I try to just open myfile.txt, this will fail. The correct path would be writing/myfile.txt. It is also possible (like with NameError) that you just made a typo.

A related issue can occur if you use the “read” flag instead of the “write” flag. Python will not give you an error if you try to open a file for writing when the file does not exist. However, if you meant to open a file for reading, but accidentally opened it for writing, and then try to read from it, you will get an UnsupportedOperation error telling you that the file was not opened for reading:

file_handle = open('myfile.txt', 'w')
file_handle.read()

---------------------------------------------------------------------------
UnsupportedOperation                      Traceback (most recent call last)
<ipython-input-15-b846479bc61f> in <module>()
      1 file_handle = open('myfile.txt', 'w')
----> 2 file_handle.read()

UnsupportedOperation: not readable

These are the most common errors with files, though many others exist. If you get an error that you’ve never seen before, searching the Internet for that error type often reveals common reasons why you might get that error.

Key Points

• Tracebacks can look intimidating, but they give us a lot of useful information about what went wrong in our program, including where the error occurred and what type of error it was.

• An error having to do with the ‘grammar’ or syntax of the program is called a SyntaxError. If the issue has to do with how the code is indented, then it will be called an IndentationError.

• A NameError will occur if you use a variable that has not been defined, either because you meant to use quotes around a string, you forgot to define the variable, or you just made a typo.

• Containers like lists and strings will generate errors if you try to access items in them that do not exist. This type of error is called an IndexError.

• Trying to read a file that does not exist will give you an FileNotFoundError. Trying to read a file that is open for writing, or writing to a file that is open for reading, will give you an IOError.