3.8. 👩💻 Know Your Error Messages¶
Many problems in your program will lead to an error message. For example as I was writing and testing this chapter of the book I wrote the following version of the example program in the previous section.
current_time_str = input("What is the current time (in hours 0-23)?") wait_time_str = input("How many hours do you want to wait") current_time_int = int(current_time_str) wait_time_int = int(wait_time_int) final_time_int = current_time_int + wait_time_int print(final_time_int)
Can you see what is wrong, just by looking at the code? Maybe, maybe not. Our brain tends to see what we think is there, so sometimes it is very hard to find the problem just by looking at the code. Especially when it is our own code and we are sure that we have done everything right!
Let’s try the program again, but this time in an activecode:
Aha! Now we have an error message that might be useful. The name error tells us
wait_time_int is not defined. It also tells us that the error is on line 5.
That’s really useful information. Now look at line five and you will see that
wait_time_int is used on both the left and the right hand side of the assignment
The error descriptions you see in activecode may be different (and more understandable!) than in a regular Python interpreter. The interpreter in activecode is limited in many ways, but it is intended for beginners, including the wording chosen to describe errors.
- You cannot use a variable on both the left and right hand sides of an assignment statement.
- No, You can, as long as all the variables on the right hand side already have values.
- wait_time_int does not have a value so it cannot be used on the right hand side.
- Yes. Variables must already have values in order to be used on the right hand side.
- This is not really an error, Python is broken.
- No, No, No!
Which of the following explains why
wait_time_int = int(wait_time_int) is an error?
In writing and using this book over the last few years we have collected a lot of statistics about the programs in this book. Here are some statistics about error messages for the exercises in this book.
Nearly 90% of the error messages encountered for this problem are SyntaxError, TypeError, NameError, ValueError, or AttributeError. We will look at these errors in three stages:
First we will define what these four error messages mean.
Then, we will look at some examples that cause these errors to occur.
Finally we will look at ways to help uncover the root cause of these messages.
Syntax errors happen when you make an error in the syntax of your program. Syntax errors are like making grammatical errors in writing. If you don’t use periods and commas in your writing then you are making it hard for other readers to figure out what you are trying to say. Similarly Python has certain grammatical rules that must be followed or else Python can’t figure out what you are trying to say.
Usually SyntaxErrors can be traced back to missing punctuation characters, such as parentheses, quotation marks, or commas. Remember that in Python commas are used to separate parameters to functions. Paretheses must be balanced, or else Python thinks that you are trying to include everything that follows as a parameter to some function.
Here are a couple examples of Syntax errors in the example program we have been using. See if you can figure out what caused them.
Find and fix the error in the following code.
current_time_str = input("What is the current time (in hours 0-23)?") wait_time_str = input("How many hours do you want to wait" current_time_int = int(current_time_str) wait_time_int = int(wait_time_str) final_time_int = current_time_int + wait_time_int print(final_time_int)
Since the error message points us to line 4 this might be a bit confusing. If
you look at line four carefully you will see that there is no problem with the
syntax. So, in this case the next step should be to back up and look at the
previous line. In this case if you look at line 2 carefully you will see that
there is a missing right parenthesis at the end of the line. Remember that
parentheses must be balanced. Since Python allows statements to continue over
multiple lines inside parentheses python will continue to scan subsequent
lines looking for the balancing right parenthesis. However in this case it
finds the name
current_time_int and it will want to interpret that as
another parameter to the input function. But, there is not a comma to
separate the previous string from the variable so as far as Python is
concerned the error here is a missing comma. From your perspective its a
Finding Clues How can you help yourself find these problems? One trick that can be
very valuable in this situation is to simply start by commenting out the line number
that is flagged as having the error. If you comment out line four, the error message
now changes to point to line 5. Now you ask yourself, am I really that bad that I
have two lines in a row that have errors on them? Maybe, so taken to the extreme, you
could comment out all of the remaining lines in the program. Now the error message
TokenError: EOF in multi-line statement This is a very technical way
of saying that Python got to the end of file (EOF) while it was still looking for
something. In this case a right parenthesis.
Find and fix the error in the following code.
current_time_str = input("What is the "current time" (in hours 0-23)?") wait_time_str = input("How many hours do you want to wait") current_time_int = int(current_time_str) wait_time_int = int(wait_time_str) final_time_int = current_time_int + wait_time_int print(final_time_int)
The error message points you to line 1 and in this case that is exactly where
the error occurs. In this case your biggest clue is to notice the difference
in highlighting on the line. Notice that the words “current time” are a
different color than those around them. Why is this? Because “current time”
is in double quotes inside another pair of double quotes Python thinks that
you are finishing off one string, then you have some other names and finally
another string. But you haven’t separated these names or strings by commas,
and you haven’t added them together with the concatenation operator (+). So,
there are several corrections you could make. First you could make the
argument to input be as follows:
"What is the 'current time' (in hours 0-23)
" Notice that here we have correctly used single quotes inside double quotes
. Another option is to simply remove the extra double quotes. Why were you
quoting “current time” anyway?
"What is the current time (in hours 0-23)"
Finding Clues If you follow the same advice as for the last problem, comment out
line one, you will immediately get a different error message. Here’s where you need
to be very careful and not panic. The error message you get now is:
'current_time_str' is not defined on line 4. You might be very tempted to think
that this is somehow related to the earlier problem and immediately conclude that
there is something wrong with the variable name
current_time_str but if you
reflect for a minute you will see that by commenting out line one you have caused a
new and unrelated error. That is you have commented out the creation of the name
current_time_str. So of course when you want to convert it to an
int you will
get the NameError. Yes, this can be confusing, but it will become much easier with
experience. It’s also important to keep calm, and evaluate each new clue carefully so
you don’t waste time chasing problems that are not really there.
Uncomment line 1 and you are back to the SyntaxError. Another track is to eliminate a
possible source of error. Rather than commenting out the entire line you might just
try to assign
current_time_str to a constant value. For example you might make
line one look like this:
current_time_str = "10" #input("What is the "current
time" (in hours 0-23)?"). Now you have assigned
current_time_str to the string
10, and commented out the input statement. And now the program works! So you
conclude that the problem must have something to do with the input function.
TypeErrors occur when you you try to combine two objects that are not compatible. For example you try to add together an integer and a string. Usually type errors can be isolated to lines that are using mathematical operators, and usually the line number given by the error message is an accurate indication of the line.
Here’s an example of a type error created by a Polish learner. See if you can find and fix the error.
In finding this error there are few lessons to think about. First, you may find it very disconcerting that you cannot understand the whole program. Unless you speak Polish then this won’t be an issue. But, learning what you can ignore, and what you need to focus on is a very important part of the debugging process. Second, types and good variable names are important and can be very helpful. In this case a and x are not particularly helpful names, and in particular they do not help you think about the types of your variables, which as the error message implies is the root of the problem here. The rest of the lessons we will get back to in a minute.
The error message provided to you gives you a pretty big hint.
TypeError: unsupported operand type(s) for FloorDiv: 'str' and 'number' on line: 5
On line five we are trying to use integer division on x and 24. The error
message tells you that you are tyring to divide a string by a number. In this
case you know that 24 is a number so x must be a string. But how? You can
see the function call on line 3 where you are converting x to an integer.
int(x) or so you think. This is lesson three and is one of the most
common errors we see in introductory programming. What is the difference
x = int(x)
int(x)converts the string referenced by x to an integer but it does not store it anywhere. It is very common to assume that
int(x)somehow changes x itself, as that is what you are intending! The thing that makes this very tricky is that
int(x)is a valid expression, so it doesn’t cause any kind of error, but rather the error happens later on in the program.
The assignment statement
x = int(x)is very different. Again, the
int(x)expression converts the string referenced by x to an integer, but this time it also changes what x references so that x now refers to the integer value returned by the
So, the solution to this problem is to change lines 3 and 4 so they are assignment statements.
Finding Clues One thing that can help you in this situation is to print out the
values and the types of the variables involved in the statement that is causing the
error. You might try adding a print statement after line 4
print(x, type(x)) You
will see that at least we have confirmed that x is of type string. Now you need to
start to work backward through the program. You need to ask yourself, where is x used
in the program? x is used on lines 2, 3, and of course 5 and 6 (where we are getting
an error). So maybe you move the print statement to be after line 2 and again after 3.
Line 3 is where you expect the value of x to be changed to an integer. Could line 4
be mysteriously changing x back to a string? Not very likely. So the value and type
of x is just what you would expect it to be after line 2, but not after line 3. This
helps you isolate the problem to line 3. In fact if you employ one of our earlier
techniques of commenting out line 3 you will see that this has no impact on the error,
and is a big clue that line 3 as it is currently written is useless.
Name errors almost always mean that you have used a variable before it has a value. Often NameErrors are simply caused by typos in your code. They can be hard to spot if you don’t have a good eye for catching spelling mistakes. Other times you may simply mis-remember the name of a variable or even a function you want to call. You have seen one example of a NameError at the beginning of this section. Here is another one. See if you can get this program to run successfully:
In this example, the student seems to be a fairly bad speller, as there are a
number of typos to fix. The first one is identified as wait_time is not
defined on line 6. Now in this example you can see that there is
str_wait_time on line 2, and
wai_time on line 4 and
line 6. If you do not have very sharp eyes its easy to miss that there is a
typo on line 4.
Finding Clues With name errors one of the best things you can do is use the editor, or browser search function. Quite often if you search for the exact word in the error message one of two things will happen:
1. The word you are searching for will appear only once in your code, it’s also likely that it will be on the right hand side of an assignment statement, or as a parameter to a function. That should confirm for you that you have a typo somewhere. If the name in question is what you thought it should be then you probably have a typo on the left hand side of an assignment statement on a line before your error message occurs. Start looking backward at your assignment statements. In some cases it’s really nice to leave all the highlighted strings from the search function visible as they will help you very quickly find a line where you might have expected your variable to be highlighted.
2. The second thing that may happen is that you will be looking directly at a line where you expected the search to find the string in question, but it will not be highlighted. Most often that will be the typo right there.
Here is another one for you to try:
This one is once again a typo, but the typo is not in a variable name, but
rather, the name of a function. The search strategy would help you with this
one easily, but there is another clue for you as well. The editor in the
textbook, as well as almost all Python editors in the world provide you with
color clues. Notice that on line 2 the function
imt is not highlighted
blue like the word
int on line 4.
And one last bit of code to fix.
In this example the error message is about
set_time not defined on line 3.
In this case the undefined name is not used in an assignment statement, but is
used as a parameter (incorrectly) to a function call. A search on
reveals that in fact it is only used once in the program. Did the author mean
set_alarm? If we make that assumption we immediately get another error
NameError: name 'alarm_time' is not defined on line: 3. The variable
alarm_time is defined on line 4, but that does not help us on line 3.
Furthermore we now have to ask the question is this function call
int(present_time, set_alarm, alarm_time) even the correct use of the
int function? The answer to that is a resounding no. Let’s list all of the
things wrong with line 3:
set_timeis not defined and never used, the author probably meant
alarm_timecannot be used as a parameter before it is defined, even on the next line!
intcan only convert one string to an integer at a time.
intshould be used in an assignment statement. Even if
intwas called with the correct number of parameters it would have no real effect.
Value errors occur when you pass a parameter to a function and the function is expecting a certain limitations on the values, and the value passed is not compatible. We can illustrate that with this particular program in two different ways.
Run the program but instead of typing in anything to the dialog box just click OK. You
should see the following error message:
ValueError: invalid literal for int() with
base 10: '' on line: 4 This error is not because you have made a mistake in your
program. Although sometimes we do want to check the user input to make sure its valid,
but we don’t have all the tools we need for that yet. The error happens because the
user did not give us something we can convert to an integer, instead we gave it an
empty string. Try running the program again. Now this time enter “ten” instead of
the number 10. You will get a similar error message.
ValueErrors are not always caused by user input error, but in this program that is the case. We’ll look again at ValueErrors again when we get to more complicated programs. For now it is worth repeating that you need to keep track of the restrictions needed for your variables, and understand what your function is expecting. You can do this by writing comments in your code, or by naming your variables in a way that reminds you of their proper form.