The previous 3 articles explained how to do profiling with 3 tools (timeit, line_profiler and cProfile). Now we will explain how to debug Python code.
In this case we will debug code using ipython.
Besides allowing profiling, ipython also allows code debugging.
The code below generates an array, then prints the array, the value of one element of that array, and iterates over the array's elements:
#!/usr/bin/env python
# -*- coding: utf-8 -*-
#Se importa numpy como np
import numpy as np
a = np.arange(10)
print a
print a[8]
print a[9]
for i in a:
print i
print a[10]The result of running the script is the following:
ecrespo@jewel:~/proyectos/ernesto-ecrespo.blogspot/debugipython$ python arreglo.py
[0 1 2 3 4 5 6 7 8 9]
8
9
0
1
2
3
4
5
6
7
8
9
Traceback (most recent call last):
File "arreglo.py", line 17, in <module>
print a[10]
IndexError: index out of boundsThe error message appears when trying to print the value of element 10 of the array a (which does not exist).
Next, run ipython from the Linux console. ipython enters an interactive interpreter, where you run the command %run script.py. This starts debug mode with the script passed as the argument:
ecrespo@jewel:~/proyectos/ernesto-ecrespo.blogspot/debugipython$ ipython
Python 2.7.3rc2 (default, Apr 22 2012, 22:35:38)
Type "copyright", "credits" or "license" for more information.
IPython 0.13.1 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.
In [1]: %run arreglo.py
[0 1 2 3 4 5 6 7 8 9]
8
9
0
1
2
3
4
5
6
7
8
9
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
/usr/lib/python2.7/dist-packages/IPython/utils/py3compat.pyc in execfile(fname, *where)
176 else:
177 filename = fname
--> 178 __builtin__.execfile(filename, *where)
/home/ecrespo/proyectos/ernesto-ecrespo.blogspot/debugipython/arreglo.py in <module>()
15 print i
16
---> 17 print a[10]
IndexError: index out of bounds
The result is the execution of the script, and it points to the section where the error is.
Now run the %debug command:
In [2]: %debug
> /home/ecrespo/proyectos/ernesto-ecrespo.blogspot/debugipython/arreglo.py(17)<module>()
15 print i
16
---> 17 print a[10]It is clear that the error is on line 17 of the script, where it tries to print element 10 of the array, which does not exist.
You can list the code by running the list command:
ipdb> list
12 print a[9]
13
14 for i in a:
15 print i
16
---> 17 print a[10]Evaluating code. You can run a Python statement to evaluate the size of the array.
ipdb> len(a)
10Print the value of a:
ipdb> print a
[0 1 2 3 4 5 6 7 8 9]Viewing the stack calls: it is a stack that holds information about the active functions of a running program. The command to see the stack is bt:
ipdb> bt
/usr/lib/python2.7/dist-packages/IPython/utils/py3compat.py(178)execfile()
174 if isinstance(fname, unicode):
175 filename = fname.encode(sys.getfilesystemencoding())
176 else:
177 filename = fname
--> 178 __builtin__.execfile(filename, *where)
> /home/ecrespo/proyectos/ernesto-ecrespo.blogspot/debugipython/arreglo.py(17)<module>()
13
14 for i in a:
15 print i
16
---> 17 print a[10]
Moverse hacia arriba en el stack se logra al ejecutar el comando `u`:
```python
ipdb> u
> /usr/lib/python2.7/dist-packages/IPython/utils/py3compat.py(178)execfile()
176 else:
177 filename = fname
--> 178 __builtin__.execfile(filename, *where)
Para moverse hacia abajo en el stack se logra al ejecutar el comando d:
ipdb> d
> /home/ecrespo/proyectos/ernesto-ecrespo.blogspot/debugipython/arreglo.py(17)<module>()
15 print i
16
---> 17 print a[10]To exit debug mode run the command exit; to leave the ipython command interpreter run exit again.
It is clear that the error in the script is on line 17, where it tries to print an element of the array that does not exist.
This is how you can debug Python code with ipython.
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