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Notes/08_Testing_debugging/03_Debugging.md

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+# 8.3 Debugging
+
+### Debugging Tips
+
+So, you're program has crashed...
+
+```bash
+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?!
+
+### Reading Tracebacks
+
+The last line is the specific cause of the crash.
+
+```bash
+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.*
+
+### Using the REPL
+
+Use the option `-i` to keep Python alive when executing a script.
+
+```bash
+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.
+
+### Debugging with Print
+
+`print()` debugging is quite common.
+
+*Tip: Make sure you use `repr()`*
+
+```python
+def spam(x):
+    print('DEBUG:', repr(x))
+    ...
+```
+
+`repr()` shows you an accurate representation of a value. Not the *nice* printing output.
+
+```python
+>>> from decimal import Decimal
+>>> x = Decimal('3.4')
+# NO `repr`
+>>> print(x)
+3.4
+# WITH `repr`
+>>> print(repr(x))
+Decimal('3.4')
+>>>
+```
+
+### The Python Debugger
+
+You can manually launch the debugger inside a program.
+
+```python
+def some_function():
+    ...
+    breakpoint()      # Enter the debugger (Python 3.7+)
+    ...
+```
+
+This starts the debugger at the `breakpoint()` call.
+For earlier Python versions:
+
+```python
+import pdb
+...
+pdb.set_trace()       # Instead of `breakpoint()`
+...
+```
+
+### Run under debugger
+
+You can also run an entire program under debugger.
+
+```bash
+bash % python3 -m pdb someprogram.py
+```
+
+It will automatically enter the debugger before the first statement. Allowing you to set breakpoints and change the configuration.
+
+Common debugger commands:
+
+```code
+(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.
+
+```code
+(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
+```
+