Debugging All Versions
This draft deletes the entire topic.
Examples
-
First, you need to install django-debug-toolbar:
pip install django-debug-toolbarNext, include it to project's installed apps, but be careful - it's always a good practice to use a different
settings.pyfile for such development-only apps and middlewares as debug toolbar:# If environment is dev... DEBUG = True INSTALLED_APPS += [ 'debug_toolbar', ]Debug toolbar also relies on static files, so appropriate app should be included as well:
INSTALLED_APPS = [ # ... 'django.contrib.staticfiles', # ... ] STATIC_URL = '/static/' # If environment is dev... DEBUG = True INSTALLED_APPS += [ 'debug_toolbar', ]Collect toolbar's static after installation:
python manage.py collectstaticThat's it, debug toolbar will appear on you project's pages, providing various useful information about execution time, SQL, static files, signals, etc.
Also,
django-debug-toolbarrequires a Content-type oftext/html,<html>and<body>tags to render properly. -
Most basic Django debugging tool is pdb, a part of Python standard library.
Init view script
Let's examine a simple
views.pyscript:from django.http import HttpResponse def index(request): foo = 1 bar = 0 bug = foo/bar return HttpResponse("%d goes here." % bug)Console command to run server:
python manage.py runserverIt's obvious that Django would throw a
ZeroDivisionErrorwhen you try to load index page, but if we'll pretend that the bug is very deep in the code, it could get really nasty.Setting a breakpoint
Fortunately, we can set a breakpoint to trace down that bug:
from django.http import HttpResponse # Pdb import import pdb; def index(request): foo = 1 bar = 0 # This is our new breakpoint pdb.set_trace() bug = foo/bar return HttpResponse("%d goes here." % bug)Console command to run server with pdb:
python -m pdb manage.py runserverNow on page load breakpoint will trigger (Pdb) prompt in the shell, which will also hang your browser in pending state.
Debugging with pdb shell
It's time to debug that view by interacting with script via shell:
> ../views.py(12)index() -> bug = foo/bar # input 'foo/bar' expression to see division results: (Pdb) foo/bar *** ZeroDivisionError: division by zero # input variables names to check their values: (Pdb) foo 1 (Pdb) bar 0 # 'bar' is a source of the problem, so if we set it's value > 0... (Pdb) bar = 1 (Pdb) foo/bar 1.0 # exception gone, ask pdb to continue execution by typing 'c': (Pdb) c [03/Aug/2016 10:50:45] "GET / HTTP/1.1" 200 111In the last line we see that our view returned an
OKresponse and executing as it should.To stop pdb loop, just input
qin a shell. -
I am downvoting this example because it is...
Remarks
Pdb
Pdb can also print out all existing variables in global or local scope, by typing globals() or locals() in (Pdb) prompt respectively.
Using Django Debug Toolbar
First, you need to install django-debug-toolbar:
pip install django-debug-toolbar
Next, include it to project's installed apps, but be careful - it's always a good practice to use a different settings.py file for such development-only apps and middlewares as debug toolbar:
# If environment is dev...
DEBUG = True
INSTALLED_APPS += [
'debug_toolbar',
]
Debug toolbar also relies on static files, so appropriate app should be included as well:
INSTALLED_APPS = [
# ...
'django.contrib.staticfiles',
# ...
]
STATIC_URL = '/static/'
# If environment is dev...
DEBUG = True
INSTALLED_APPS += [
'debug_toolbar',
]
Collect toolbar's static after installation:
python manage.py collectstatic
That's it, debug toolbar will appear on you project's pages, providing various useful information about execution time, SQL, static files, signals, etc.
Also, django-debug-toolbar requires a Content-type of text/html, <html> and <body> tags to render properly.
Using Python Debugger (Pdb)
Most basic Django debugging tool is pdb, a part of Python standard library.
Init view script
Let's examine a simple views.py script:
from django.http import HttpResponse
def index(request):
foo = 1
bar = 0
bug = foo/bar
return HttpResponse("%d goes here." % bug)
Console command to run server:
python manage.py runserver
It's obvious that Django would throw a ZeroDivisionError when you try to load index page, but if we'll pretend that the bug is very deep in the code, it could get really nasty.
Setting a breakpoint
Fortunately, we can set a breakpoint to trace down that bug:
from django.http import HttpResponse
# Pdb import
import pdb;
def index(request):
foo = 1
bar = 0
# This is our new breakpoint
pdb.set_trace()
bug = foo/bar
return HttpResponse("%d goes here." % bug)
Console command to run server with pdb:
python -m pdb manage.py runserver
Now on page load breakpoint will trigger (Pdb) prompt in the shell, which will also hang your browser in pending state.
Debugging with pdb shell
It's time to debug that view by interacting with script via shell:
> ../views.py(12)index()
-> bug = foo/bar
# input 'foo/bar' expression to see division results:
(Pdb) foo/bar
*** ZeroDivisionError: division by zero
# input variables names to check their values:
(Pdb) foo
1
(Pdb) bar
0
# 'bar' is a source of the problem, so if we set it's value > 0...
(Pdb) bar = 1
(Pdb) foo/bar
1.0
# exception gone, ask pdb to continue execution by typing 'c':
(Pdb) c
[03/Aug/2016 10:50:45] "GET / HTTP/1.1" 200 111
In the last line we see that our view returned an OK response and executing as it should.
To stop pdb loop, just input q in a shell.
