[FIXED] How to debug in Python: Why doesn't pdb descend into function call?

Issue

I've tracked down a bug in which io.open() should have been passed 'utf-8' instead of 'utf8'. Minimal executable code below. Why doesn't the IPython traceback indicate a line number, and why does pdb neither report that the error was with the io.open function call nor report anything from within the the io.open code? What could I have done with pdb or the IPython debugger or the Canopy debugger layered on top of it to have had an easier time debugging this one?

Checking my IPython version is also confusing. The Canopy package manager reports that both ipython 4.0.0-3 and ipython4 4.0.0-9 are installed, but import IPython followed by IPython.version_info evaluates to (2, 4, 1, '').

my_module.py in Canopy code editor:

import io
def my_function(filename):
    with io.open(my_other_function(filename), u'r', u'utf8')
def my_other_function(text):
    return u'modified' + text

In the IPython session:

In []: import pdb
In []: import my_module
In []: my_module.my_function(filename)

-------------------------------------------------------------------------
TypeError                 Traceback (most recent call last)
<ipython-input-5-4c50d9f6cb5c> in <module>()
----> 1 my_module.my_function(filename)

C:\my_module in my_function(filename)

TypeError: an integer is required 

In []: pdb.pm()
> c:\users\bbrown\documents\github\canvasapi-python\capi\my_module.py(3)my_function()
-> with io.open(my_other_function(filename), u'w', u'utf8') as filehandle:

(Pdb) up
> <ipython-input-14-f6d6cc2c1670>(1)<module>()
-> my_module.my_function('testjunk')

(Pdb) down
> c:\users\bbrown\documents\github\canvasapi-python\capi\my_module.py(3)my_function()
-> with io.open(my_other_function(filename), u'w', u'utf8') as filehandle:

(Pdb) args
filename = testjunk

(Pdb) down
*** Newest frame

Given that 'utf-8' works fine as an argument, the TypeError is surprising unless originating from within the code of open, yet the call to open was not placed on the stack, at least not as navigable from pdb. Thank you for helping me and others learn how to debug more efficiently!

Solution

io.open is a builtin function:

In [8]: import io

In [9]: type(io.open)
Out[9]: builtin_function_or_method

It's not written in Python, so there's nothing for the debugger to debug. Your error is caused by improper arguments being passed into io.open:

open(file, mode='r', buffering=-1, encoding=None,
     errors=None, newline=None, closefd=True, opener=None) -> file object

You passed 'utf-8' as the third argument, but since buffering is supposed to be an integer, the function raised a descriptive TypeError. You can fix it by making encoding a keyword argument:

io.open(filename, mode='r', encoding='utf8')

Also, you don't need to import the io module explicitly. The open builtin is exactly the same function:

In [15]: open
Out[15]: <function io.open>

Answered By - Blender