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