CSE 101: Introduction to Computational and Algorithmic Thinking
Debugging Tips and Techniques
This debugging guide is courtesy of Dr. John DeNero and the CS 61A course staff at UC Berkeley.
By now, you will have encountered various bugs when programming for this class. Most often, you will try to run your code and see something like this:
Traceback (most recent call last): File "<pyshell#29>", line 3 in <module> result = buggy(5) File <pyshell#29>", line 5 in buggy return f + x TypeError: unsupported operand type(s) for +: 'function' and 'int'
This is called a traceback message. It prints out the chain of function calls that led up to the error, with the most recent function call at the bottom. You can follow this chain to figure out which function(s) caused the problem.
Notice that the lines in the traceback appear to be paired together. The first line in such a pair has the following format:
File "<file name>", line <number>, in <function>
That line provides you with the following information:
- File name: the name of the file that contains the problem.
- Number: the line number in the file that cuased the problem, or the line number that contains the next function call
- Function: the name of the function in which the line can be found.
The second line in the pair (it's indented farther in than the first) displays the actual line of code that makes the next function call. This gives you a quick look at what arguments were passed into the function, in what context the function was being used, etc.
Finally, remember that the traceback is organized with the "most recent call last."
The very last line in the traceback message is the error statement. An error statement has the following format:
<error type>: <error message>
This line provides you with two pieces of information:
- Error type: the type of error that was caused (e.g.
TypeError). These are usually descriptive enough to help you narrow down your search for the cause of error.
- Error message: a more detailed description of exactly what caused the error. Different error types produce different error messages.
When you first learn how to program, it can be hard to spot bugs in
your code. One common practice is to add
foo keeps returning the
def foo(x): result = some_function(x) return result // 5
We can add a print statement before the return to check what
some_function is returning:
def foo(x): result = some_function(x) print('result is', result) return other_function(result)
If it turns out
result is not what we expect it to be, we would go
some_function to see if it works properly. Otherwise, we
might have to add a print statement before the return to check
def foo(x): result = some_function(x) print('result is', result) tmp = other_function(result) print('other_function returns', tmp) return tmp
Don't just print out a variable -- add some sort of message to make it easier for you to read:
print(tmp) # harder to keep track print('tmp was this:', tmp) # easier
whileloop to view the state of the counter variables after each iteration:
i = 0 while i < n: i += func(i) print('i is', i)
- Don't just put random
However, sometimes we would like to leave the debugging code if we
need to periodically test our file. It can get kind of annoying if
every time we run our file, debugging messages pop up. One way to
avoid this is to use a global
debug = True def foo(n): i = 0 while i < n: i += func(i) if debug: print('i is', i)
Now, whenever we want to do some debugging, we can set the global
debug variable to
True, and when we don't want to see any
debugging input, we can turn it to
False (such a variable is called
The following are common error types that Python programmers run into.
- Cause: code syntax mistake
File "file name", line number def incorrect(f) ^ SyntaxError: invalid syntax
- Solution: the
^symbol points to the code that contains invalid syntax. The error message doesn't tell you what is wrong, but it does tell you where.
- Notes: Python will check for
SyntaxErrorsbefore executing any code. This is different from other errors, which are only raiased during runtime.
- Cause: improper indentation
File "file name", line number print('improper indentation') IndentationError: unindent does not match any outer indentation level
- Solution: The line that is improperly indented is displayed. Simply re-indent it.
- Notes: If you are inconsistent with tabs and spaces, Python will raise one of these. Make sure you use either spaces or tabs, not both!
- Invalid operand types for primitive operators. You are probably trying to add/subract/multiply/divide incompatible types.
TypeError: unsupported operand type(s) for +: 'function' and 'int'
- Using non-function objects in function calls.
3 > square(3) Traceback (most recent call last): ... TypeError: 'int' object is not callable> square =
- Passing an incorrect number of arguments to a function.
>>> add(3) Traceback (most recent call last): ... TypeError: add expected 2 arguments, got 1
- Cause: variable not assigned to anything OR it doesn't exist. This includes function names.
File "file name", line number y = x + 3 NameError: global name 'x' is not defined
- Solution: Make sure you are initializing the variable (i.e. assigning the variable to a value) before you use it.
- Notes: The reason the error message says "global name" is because Python will start searching for the variable from a function's local frame. If the variable is not found there, Python will keep searching the parent frames until it reaches the global frame. If it still can't find the variable, Python raises the error.
- Cause: trying to index a sequence (e.g. a tuple, list, string) with a number that exceeds the size of the sequence.
File "file name", line number x IndexError: tuple index out of range
- Solution: Make sure the index is within the bounds of the
sequence. If you're using a variable as an index (e.g.
seq[x], make sure the variable is assigned to a proper index.
Python is case sensitive. The variable
hello is not the same as
helo. This will usually show up as a
NameError, but sometimes
misspelled variables will actually have been defined. In that case, it can be
difficult to find errors, and it is never gratifying to discover it's just a
A common bug is to leave off the closing parenthesis. This will show up as a
SyntaxError. Consider the following code:
def fun(): return foo(bar() # missing a parenthesis here fun()
Python will raise a
SyntaxError, but will point to the line after the missing parenthesis:
File "file name", line "number" fun() ^ SyntaxError: invalid syntax
In general, if Python points a
SyntaxError to a seemingly correct
line, you are probably forgetting a parenthesis somewhere.
Missing close quotes
This is similar to the previous bug, but much easier to catch. Python will actually tell you the line that is missing the quote:
File "file name", line "number" return 'hi ^ SyntaxError: EOL while scanning string literal
EOL stands for "End of Line."
The single equal sign
= is used for assignment; the double equal sign
== is used for testing equivalence. The most common error of this form is
if x = 3:
Infinite loops are often caused by
while loops whose conditions never change.
i = 0 while i < 10: print(i)
Sometimes you might have incremented the wrong counter:
i, n = 0, 0 while i < 10: print(i) n += 1
while loop or recursive function might stop one iteration too
short. Here, it's best to walk through the iteration/recursion to see what
number the loop stops at.