COMP 1000: Think Like a Computer: The PreTeXt Interactive Edition

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 that 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 statement.

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.

The vast majority of the error messages encountered for this problem are SyntaxError, TypeError, NameError, or ValueError. We will look at these errors in three stages:

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. Parentheses 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.

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)

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 4, 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 changes to:

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.

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)

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:

NameError: name '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 1 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 1 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.

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 this print statement after line 4:

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:

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:

And one last bit of code to fix.

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:

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.