Develop Reference

Why does error traceback show edited script instead of what actually ran?

Background
Consider the following minimal example:
When I save the following script and run it from terminal,
import time
time.sleep(5)
raise Exception
the code will raise an error after sleeping five seconds, leaving the following traceback.
Traceback (most recent call last):
File "test/minimal_error.py", line 4, in <module>
raise Exception
Exception
Now, say, I run the script, and during the 5-second-sleep, I add a line in the middle.
import time
time.sleep(5)
a = 1
raise Exception
After the python interpreter wakes up from the sleep and reaches the next line, raise Exception, it will raise the error, but it leaves the following traceback.
Traceback (most recent call last):
File "test/minimal_error.py", line 4, in <module>
a = 1
Exception
So the obvious problem is that it doesn't print the actual code that caused the error. Although it gives the correct line number (correctly reflecting the version of the script that is running, while understandably useless) and a proper error message, I can't really know what piece of code actually caused the error.
In real practice, I implement one part of a program, run it to see if that part is doing fine, and while it is still running, I move on to the next thing I have to implement. And when the script throws an error, I have to find which actual line of code caused the error. I usually just read the error message and try to deduce the original code that caused it. Sometimes it isn't easy to guess, so I copy the script to clipboard and rollback the code by undoing what I've written after running the script, check the line that caused error, and paste back from clipboard.
Question
Is there any understandable reason why the interpreter shows a = 1, which is line 4 of the "current" version of the code, instead of raise Exception, which is line 4 of the "running" version of the code? If the interpreter knows "line 4" caused the error and the error message is "Exception", why can't it say the command raise Exception raised it?
I'm not really sure if this question is on-topic here, but I don't think I can conclude it off-topic from what the help center says. It is about "[a] software [tool] commonly used by programmers" (the Python interpreter) and is "a practical, answerable problem that is unique to software development," I think. I don't think it's opinion-based, because there should be a reason for this choice of implementation.
(Observed the same in Python 2.7.16, 3.6.8, 3.7.2, and 3.7.3, so it doesn't seem to be version-specific, but a thing that just happens in Python.)

The immediate reason is that Python re-opens the file and reads the specified line again to print it in error messages. So why would it need to do that when it already read the file in the beginning? Because it doesn't keep the source code in memory, only the generated byte code.
In fact, Python will never hold the entire contents of the source file in memory at one time. Instead the lexer will read from the file and produce one token at a time, which the parser then parses and turns into byte code. Once the parser is done with a token, it's gone.
So the only way to get back at the original source code is to open the source file again.

I think it a classic problem which is described here.
Sleep use os system call to pause execution of that thread.

Python traceback broken: code out of sync with what is executed?

I'm running into a strange situation simply running a script from the command-line: when I run into errors, often the traceback given contains code that makes no sense. Every line in the traceback should be a method call, but sometimes they aren't. Also, the lines referenced in the traceback don't correspond to the given error. What's happening? As an example, here is a simple error where numpy wasn't imported, but the traceback makes little sense and refers to unrelated code lines:
Traceback (most recent call last):
File "bin/train_global_model.py", line 549, in <module>
if __name__ == '__main__':
File "bin/train_global_model.py", line 236, in main
def main():
File "bin/train_global_model.py", line 407, in do_training
tb_writer=train_writer,
File "bin/train_global_model.py", line 200, in run_iteration
print(accuracy)
NameError: global name 'np' is not defined
Pay special attention to the code lines referenced.
Is python caching code that is executed somewhere but then referring to the actual file when it's tracing an exception? Running Python 2.7.13.

Python saves line numbers not the actual source code, when running programs. For tracebacks it loads the source code and shows the corresponding lines to the numbers. When the source changes, while the program is running, you get lines out of sync.

Possible to modify built-in error messages in Python? (If so, how?)

I am wondering if it is possible to edit/customize the behavior and printout of built-in errors in Python. For example, if I type:
>>> a = 1
>>> print A
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'A' is not defined
I want the output to instead be:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'A' is not defined. Check capitalization.
Moreover, I want this to occur at a global level, for ALL FUTURE CODE, without having to explicitly include an exception in my code. If such a change is possible, I would assume this needs to be done at the very source or library-file level of Python. However, I am not sure where exactly to look to know if this is even possible.
I am using Python 2.7 on both Ubuntu and OSX, so help on either system would be appreciated.
(My apologies in advance if this is covered elsewhere, but searching for threads on "changing Python error messages" generally gave me topics on Exceptions, which is not necessarily my interest here. If anyone can point me to a page on this though, I'd greatly appreciate it.)

YES! There is a way to exactly what you want!
traceback.py is the program that detects errors in your code. It then gives you an explanation of what happened (creates the error message that you see.)
You can find this file in your library folder for python.
When in that file you can change the messages that it outputs when you come across an error!
Please tell me if this helped you!

Python: AttributeError: 'str' object has no attribute 'decompressUnknownOriginalSize'

First off, I'm not a programmer by any means. I just try to follow instructions.
So, I'm trying to use a script to decode game files. The problem is that I'm getting an error.
I've tried the script on 2 different machines wheres both give the same error. At the same time, I have friends using the exact same script w/o getting this error. Does anyone have any clue what is wrong?
Traceback (most recent call last):
File "F:\Frostbite Decoding\Decoding Files\bf4dumper.py", line 258, in <module>
if "tocRoot" in locals(): dumpRoot(tocRoot)
File "F:\Frostbite Decoding\Decoding Files\bf4dumper.py", line 249, in dumpRoot
dump(fname,targetDirectory)
File "F:\Frostbite Decoding\Decoding Files\bf4dumper.py", line 198, in dump
LZ77.decompressUnknownOriginalSize(catEntry.path,catEntry.offset,catEntry.size,targetPath)
AttributeError: 'str' object has no attribute 'decompressUnknownOriginalSize'
If you need anymore information, please let me know.

Python is saying that it is receiving a string and that there is no method for decompressUnknownOriginalSize. This sounds like the script needs to get some other data type or argument type than string. Look for where "decompressUnknownOriginalSize" is called and see what data type is being passed to that argument.

Image layering in Tkinter, Python [figured it out (smacks forehead)]

I am working with a graphics module called cTurtle, which builds upon TKinter. Each controllable 'Turtle' has an image assigned to it with TKinter. I am trying to add a method to the module that will allow me to change the layering position of a given 'Turtle'. Or to put it another way, i want to add a method that will allow me to put a given image on top of the displaylist. When i call the method i have made, i get this error
Traceback (most recent call last):
File "C:\Documents and Settings\login\Desktop\pYTHON EXPERIMENTS\canvastest.py", line 22, in <module>
bob.visUp()
File "C:\Documents and Settings\login\Desktop\pYTHON EXPERIMENTS\jTurtle.py", line 2017, in visUp
titem = self.turtle._item
File "C:\Documents and Settings\login\Desktop\pYTHON EXPERIMENTS\jTurtle.py", line 542, in _visUp
def _visUp(self, item):
File "<string>", line 1, in type
File "C:\Python32\lib\tkinter\__init__.py", line 2325, in type
return self.tk.call(self._w, 'type', tagOrId) or None
_tkinter.TclError: invalid command name ".16847312"
Does someone recognize why the function is rejecting what i passed to it? The number in quotes appears to be an object id, which should work.
EDIT
well, i did myself a great disservice here by not pasting everything involved. I coded these new methods several days ago, and have been trying to debug since. Turns out i coded them right the first time. The problem was in how i called them. I added the visup() test line after exitOnClick(), meaning that the widget had, in fact, already been destroyed.
But, hey, i coded it right the first time! sorta...