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So, you're program has crashed...
bash % python3 blah.py
Traceback (most recent call last):
File "blah.py", line 13, in?foo()
File "blah.py", line 10, in foo
bar()
File "blah.py", line 7, in bar
spam()
File "blah.py", 4, in spam
line x.append(3)
AttributeError: 'int' object has no attribute 'append'Now what?!
The last line is the specific cause of the crash.
bash % python3 blah.py
Traceback (most recent call last):
File "blah.py", line 13, in?foo()
File "blah.py", line 10, in foo
bar()
File "blah.py", line 7, in bar
spam()
File "blah.py", 4, in spam
line x.append(3)
# Cause of the crash
AttributeError: 'int' object has no attribute 'append'However, it's not always easy to read or understand.
PRO TIP: Paste the whole traceback into Google.
Use the option -i to keep Python alive when executing a script.
bash % python3 -i blah.py
Traceback (most recent call last):
File "blah.py", line 13, in?foo()
File "blah.py", line 10, in foo
bar()
File "blah.py", line 7, in bar
spam()
File "blah.py", 4, in spam
line x.append(3)
AttributeError: 'int' object has no attribute 'append'
>>>It preserves the interpreter state. That means that you can go poking around after the crash. Checking variable values and other state.
print() debugging is quite common.
Tip: Make sure you use repr()
defspam(x):
print('DEBUG:', repr(x))
...repr() shows you an accurate representation of a value. Not the nice printing output.
>>>fromdecimalimportDecimal>>>x=Decimal('3.4')
# NO `repr`>>>print(x)
3.4# WITH `repr`>>>print(repr(x))
Decimal('3.4')
>>>You can manually launch the debugger inside a program.
defsome_function():
...
breakpoint() # Enter the debugger (Python 3.7+)
...This starts the debugger at the breakpoint() call.
In earlier Python versions, you did this. You'll sometimes see this mentioned in other debugging guides.
importpdb
...
pdb.set_trace() # Instead of `breakpoint()`
...You can also run an entire program under debugger.
bash % python3 -m pdb someprogram.pyIt will automatically enter the debugger before the first statement. Allowing you to set breakpoints and change the configuration.
Common debugger commands:
(Pdb) help # Get help
(Pdb) w(here) # Print stack trace
(Pdb) d(own) # Move down one stack level
(Pdb) u(p) # Move up one stack level
(Pdb) b(reak) loc # Set a breakpoint
(Pdb) s(tep) # Execute one instruction
(Pdb) c(ontinue) # Continue execution
(Pdb) l(ist) # List source code
(Pdb) a(rgs) # Print args of current function
(Pdb) !statement # Execute statement
For breakpoints location is one of the following.
(Pdb) b 45 # Line 45 in current file
(Pdb) b file.py:45 # Line 34 in file.py
(Pdb) b foo # Function foo() in current file
(Pdb) b module.foo # Function foo() in a module
It runs. Ship it!