Understanding the Python Traceback

Python Traceback Overview

There are several sections to every Python traceback that are important. The diagram below highlights the various parts:

In Python, it’s best to read the traceback from the bottom up:

1. Blue box: The last line of the traceback is the error message line. It contains the exception name that was raised.

2. Green box: After the exception name is the error message. This message usually contains helpful information for understanding the reason for the exception being raised.

3. Yellow box: Further up the traceback are the various function calls moving from bottom to top, most recent to least recent. These calls are represented by two-line entries for each call. The first line of each call contains information like the file name, line number, and module name, all specifying where the code can be found.

4. Red underline: The second line for these calls contains the actual code that was executed.

There are a few differences between traceback output when you’re executing your code in the command-line and running code in the REPL. Below is the same code from the previous section executed in a REPL and the resulting traceback output:

>>> def greet(someone):
...   print('Hello, ' + someon)
... 
>>> greet('Chad')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in greet
NameError: name 'someon' is not defined

Notice that in place of file names, you get "<stdin>". This makes sense since you typed the code in through standard input. Also, the executed lines of code are not displayed in the traceback.

Specific Traceback Walkthrough

Going through some specific traceback output will help you better understand and see what information the traceback will give you.

The code below is used in the examples following to illustrate the information a Python traceback gives you:

# greetings.py
def who_to_greet(person):
    return person if person else input('Greet who? ')

def greet(someone, greeting='Hello'):
    print(greeting + ', ' + who_to_greet(someone))

def greet_many(people):
    for person in people:
        try:
            greet(person)
        except Exception:
            print('hi, ' + person)

Here, who_to_greet() takes a value, person, and either returns it or prompts for a value to return instead.

Then, greet() takes a name to be greeted, someone, and an optional greeting value and calls print(). who_to_greet() is also called with the someone value passed in.

Finally, greet_many() will iterate over the list of people and call greet(). If there is an exception raised by calling greet(), then a simple backup greeting is printed.

This code doesn’t have any bugs that would result in an exception being raised as long as the right input is provided.

If you add a call to greet() to the bottom of greetings.py and specify a keyword argument that it isn’t expecting (for example greet('Chad', greting='Yo')), then you’ll get the following traceback:

$ python example.py
Traceback (most recent call last):
  File "/path/to/greetings.py", line 19, in <module>
    greet('Chad', greting='Yo')
TypeError: greet() got an unexpected keyword argument 'greting'

Once again, with a Python traceback, it’s best to work backward, moving up the output. Starting at the final line of the traceback, you can see that the exception was a TypeError. The messages that follow the exception type, everything after the colon, give you some great information. It tells you that greet() was called with a keyword argument that it didn’t expect. The unknown argument name is also given to you: greting.

Moving up, you can see the line that resulted in the exception. In this case, it’s the greet() call that we added to the bottom of greetings.py.

The next line up gives you the path to the file where the code exists, the line number of that file where the code can be found, and which module it’s in. In this case, because our code isn’t using any other Python modules, we just see <module> here, meaning that this is the file that is being executed.

With a different file and different input, you can see the traceback really pointing you in the right direction to find the issue. If you are following along, remove the buggy greet() call from the bottom of greetings.py and add the following file to your directory:

# example.py
from greetings import greet

greet(1)

Here you’ve set up another Python file that is importing your previous module, greetings.py, and using greet() from it. Here’s what happens if you now run example.py:

$ python example.py
Traceback (most recent call last):
  File "/path/to/example.py", line 3, in <module>
    greet(1)
  File "/path/to/greetings.py", line 5, in greet
    print(greeting + ', ' + who_to_greet(someone))
TypeError: must be str, not int

The exception raised in this case is a TypeError again, but this time the message is a little less helpful. It tells you that somewhere in the code it was expecting to work with a string, but an integer was given.

Moving up, you see the line of code that was executed. Then the file and line number of the code. This time, however, instead of <module>, we get the name of the function that was being executed, greet().

Moving up to the next executed line of code, we see our problematic greet() call passing in an integer.

Sometimes after an exception is raised, another bit of code catches that exception and also results in an exception. In these situations, Python will output all exception tracebacks in the order in which they were received, once again ending in the most recently raise exception’s traceback.

Since this can be a little confusing, here’s an example. Add a call to greet_many() to the bottom of greetings.py:

# greetings.py
...
greet_many(['Chad', 'Dan', 1])

This should result in printing greetings to all three people. However, if you run this code, you’ll see an example of the multiple tracebacks being output:

$ python greetings.py
Hello, Chad
Hello, Dan
Traceback (most recent call last):
  File "greetings.py", line 10, in greet_many
    greet(person)
  File "greetings.py", line 5, in greet
    print(greeting + ', ' + who_to_greet(someone))
TypeError: must be str, not int

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "greetings.py", line 14, in <module>
    greet_many(['Chad', 'Dan', 1])
  File "greetings.py", line 12, in greet_many
    print('hi, ' + person)
TypeError: must be str, not int

Notice the highlighted line starting with During handling in the output above. In between all tracebacks, you’ll see this line. Its message is very clear, while your code was trying to handle the previous exception, another exception was raised.

You have seen the previous exception before, when you called greet() with an integer. Since we added a 1 to the list of people to greet, we can expect the same result. However, the function greet_many() wraps the greet() call in a try and except block. Just in case greet() results in an exception being raised, greet_many() wants to print a default greeting.

The relevant portion of greetings.py is repeated here:

def greet_many(people):
    for person in people:
        try:
            greet(person)
        except Exception:
            print('hi, ' + person)

So when greet() results in the TypeError because of the bad integer input, greet_many() handles that exception and attempts to print a simple greeting. Here the code ends up resulting in another, similar, exception. It’s still attempting to add a string and an integer.

Seeing all of the traceback output can help you see what might be the real cause of an exception. Sometimes when you see the final exception raised, and its resulting traceback, you still can’t see what’s wrong. In those cases, moving up to the previous exceptions usually gives you a better idea of the root cause.

AttributeError

The AttributeError is raised when you try to access an attribute on an object that doesn’t have that attribute defined. The Python documentation defines when this exception is raised:

Raised when an attribute reference or assignment fails. (Source)

Here’s an example of the AttributeError being raised:

>>> an_int = 1
>>> an_int.an_attribute
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'int' object has no attribute 'an_attribute'

The error message line for an AttributeError tells you that the specific object type, int in this case, doesn’t have the attribute accessed, an_attribute in this case. Seeing the AttributeError in the error message line can help you quickly identify which attribute you attempted to access and where to go to fix it.

Most of the time, getting this exception indicates that you are probably working with an object that isn’t the type you were expecting:

>>> a_list = (1, 2)
>>> a_list.append(3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'tuple' object has no attribute 'append'

In the example above, you might be expecting a_list to be of type list, which has a method called .append(). When you receive the AttributeError exception and see that it was raised when you are trying to call .append(), that tells you that you probably aren’t dealing with the type of object you were expecting.

Often, this happens when you are expecting an object to be returned from a function or method call to be of a specific type, and you end up with an object of type None. In this case, the error message line will read, AttributeError: 'NoneType' object has no attribute 'append'.

ImportError

The ImportError is raised when something goes wrong with an import statement. You’ll get this exception, or its subclass ModuleNotFoundError, if the module you are trying to import can’t be found or if you try to import something from a module that doesn’t exist in the module. The Python documentation defines when this exception is raised:

Raised when the import statement has troubles trying to load a module. Also raised when the ‘from list’ in from ... import has a name that cannot be found. (Source)

Here’s an example of the ImportError and ModuleNotFoundError being raised:

>>> import asdf
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'asdf'
>>> from collections import asdf
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: cannot import name 'asdf'

In the example above, you can see that attempting to import a module that doesn’t exist, asdf, results in the ModuleNotFoundError. When attempting to import something that doesn’t exist, asdf, from a module that does exists, collections, this results in an ImportError. The error message lines at the bottom of the tracebacks tell you which thing couldn’t be imported, asdf in both cases.

IndexError

The IndexError is raised when you attempt to retrieve an index from a sequence, like a list or a tuple, and the index isn’t found in the sequence. The Python documentation defines when this exception is raised:

Raised when a sequence subscript is out of range. (Source)

Here’s an example that raises the IndexError:

>>> a_list = ['a', 'b']
>>> a_list[3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

The error message line for an IndexError doesn’t give you great information. You can see that you have a sequence reference that is out of range and what the type of the sequence is, a list in this case. That information, combined with the rest of the traceback, is usually enough to help you quickly identify how to fix the issue.

KeyError

Similar to the IndexError, the KeyError is raised when you attempt to access a key that isn’t in the mapping, usually a dict. Think of this as the IndexError but for dictionaries. The Python documentation defines when this exception is raised:

Raised when a mapping (dictionary) key is not found in the set of existing keys. (Source)

Here’s an example of the KeyError being raised:

>>> a_dict['b']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'b'

The error message line for a KeyError gives you the key that could not be found. This isn’t much to go on but, combined with the rest of the traceback, is usually enough to fix the issue.

For an in-depth look at KeyError, take a look at Python KeyError Exceptions and How to Handle Them.

NameError

The NameError is raised when you have referenced a variable, module, class, function, or some other name that hasn’t been defined in your code. The Python documentation defines when this exception is raised:

Raised when a local or global name is not found. (Source)

In the code below, greet() takes a parameter person. But in the function itself, that parameter has been misspelled to persn:

>>> def greet(person):
...     print(f'Hello, {persn}')
>>> greet('World')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in greet
NameError: name 'persn' is not defined

The error message line of the NameError traceback gives you the name that is missing. In the example above, it’s a misspelled variable or parameter to the function that was passed in.

A NameError will also be raised if it’s the parameter that you misspelled:

>>> def greet(persn):
...     print(f'Hello, {person}')
>>> greet('World')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in greet
NameError: name 'person' is not defined

Here, it might seem as though you’ve done nothing wrong. The last line that was executed and referenced in the traceback looks good. If you find yourself in this situation, then the thing to do is to look through your code for where the person variable is used and defined. Here you can quickly see that the parameter name was misspelled.

SyntaxError

The SyntaxError is raised when you have incorrect Python syntax in your code. The Python documentation defines when this exception is raised:

Raised when the parser encounters a syntax error. (Source)

Below, the problem is a missing colon that should be at the end of the function definition line. In the Python REPL, this syntax error is raised right away after hitting enter:

>>> def greet(person)
  File "<stdin>", line 1
    def greet(person)
                    ^
SyntaxError: invalid syntax

The error message line of the SyntaxError only tells you that there was a problem with the syntax of your code. Looking into the lines above gives you the line with the problem and usually a ^ (caret) pointing to the problem spot. Here, the colon is missing from the function’s def statement.

Also, with SyntaxError tracebacks, the regular first line Traceback (most recent call last): is missing. That is because the SyntaxError is raised when Python attempts to parse your code, and the lines aren’t actually being executed.

TypeError

The TypeError is raised when your code attempts to do something with an object that can’t do that thing, such as trying to add a string to an integer or calling len() on an object where its length isn’t defined. The Python documentation defines when this exception is raised:

Raised when an operation or function is applied to an object of inappropriate type. (Source)

Following are several examples of the TypeError being raised:

>>> 1 + '1'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'
>>> '1' + 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: must be str, not int
>>> len(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'int' has no len()

All of the above examples of raising a TypeError results in an error message line with different messages. Each of them does a pretty good job of informing you of what is wrong.

The first two examples attempt to add strings and integers together. However, they are subtly different:

• The first is trying to add a str to an int.
• The second is trying to add an int to a str.

The error message lines reflect these differences.

The last example attempts to call len() on an int. The error message line tells you that you can’t do that with an int.

ValueError

The ValueError is raised when the value of the object isn’t correct. You can think of this as an IndexError that is raised because the value of the index isn’t in the range of the sequence, only the ValueError is for a more generic case. The Python documentation defines when this exception is raised:

Raised when an operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as IndexError. (Source)

Here are two examples of ValueError being raised:

>>> a, b, c = [1, 2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: not enough values to unpack (expected 3, got 2)
>>> a, b = [1, 2, 3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack (expected 2)

The ValueError error message line in these examples tells you exactly what the problem is with the values:

1. In the first example, you are trying to unpack too many values. The error message line even tells you that you were expecting to unpack 3 values but got 2 values.

2. In the second example, the problem is that you are getting too many values and not enough variables to unpack them into.