229 lines
8 KiB
Markdown
229 lines
8 KiB
Markdown
# Debugging Python code
|
|
{:.no_toc}
|
|
|
|
<nav markdown="1" class="toc-class">
|
|
* TOC
|
|
{:toc}
|
|
</nav>
|
|
|
|
## The goal
|
|
|
|
We want to debug our Python source code like professionals.
|
|
|
|
Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
|
|
|
|
## Simple debugging in cell mode
|
|
|
|
If we use cells ( # %% ) (i.e. Python Interactive window ) then we can use the Variable button in the interactive Python window to open a variable viewer.
|
|
|
|
|
|
|
|
Also you can interact with the Python kernel directly via the command input line (the lower thing in the interactive window). There you can enter a Python command and press Shift + Enter to execute it.
|
|
|
|
## Where to find the debugger (normal mode)?
|
|
|
|
|
|
|
|
## Where to find the debugger (cell mode)?
|
|
|
|
|
|
|
|
Note: My first Step Over
|
|
|
|
click is ignored. Why? I don't know.
|
|
|
|
|
|
|
|
|
|
## Debugger controls
|
|
### Control toolbar
|
|
|
|
|||
|
|
|---|---|
|
|
|| Continue -- Debugger continues to run the code until the next breakpoint is reached or until the program ends. If end of the program is reached, the debugger closes. |
|
|
|| Step Over|
|
|
|| Step Into|
|
|
|| Step Out|
|
|
|| Restart -- Starts the debugging process from the beginning. |
|
|
|| Stop -- Stops the debugging and the debugger closes.|
|
|
|
|
|
|
### Continue vs Step Over vs Step Into vs Step Out
|
|
|
|
Example code:
|
|
|
|
```python
|
|
# %%
|
|
def function(x: int) -> int:
|
|
x += 1
|
|
x += 1
|
|
x += 1
|
|
x += 1
|
|
return x
|
|
|
|
|
|
x = 1
|
|
x = function(x)
|
|
x = function(x)
|
|
x = function(x)
|
|
```
|
|
|
|
### Continue
|
|
|
|
|
|
|
|
Then  Continue:
|
|
|
|
|
|
|
|
|
|
### Step Over
|
|
* ["If the current line contains a function call, Step Over runs the code and then suspends execution at the first line of code after the called function returns."](https://docs.microsoft.com/en-us/visualstudio/debugger/navigating-through-code-with-the-debugger?view=vs-2022#BKMK_Step_over_Step_out)
|
|
|
|
|
|
|
|
Then  Step Over:
|
|
|
|
|
|
|
|
### Step Into
|
|
* ["The debugger steps through code statements, not physical lines."](https://docs.microsoft.com/en-us/visualstudio/debugger/navigating-through-code-with-the-debugger?view=vs-2022#BKMK_Step_into__over__or_out_of_the_code)
|
|
* ["But when you step into this line, the debugger treats the condition as one step and the consequence as another."](https://docs.microsoft.com/en-us/visualstudio/debugger/navigating-through-code-with-the-debugger?view=vs-2022#BKMK_Step_into__over__or_out_of_the_code)
|
|
* ["On a nested function call, Step Into steps into the most deeply nested function. For example, if you use Step Into on a call like Func1(Func2()), the debugger steps into the function Func2."](https://docs.microsoft.com/en-us/visualstudio/debugger/navigating-through-code-with-the-debugger?view=vs-2022#BKMK_Step_into__over__or_out_of_the_code)
|
|
|
|
|
|
|
|
|
|
Then  Step Into:
|
|
|
|
|
|
|
|
|
|
### Step Out
|
|
|
|
* ["Step Out continues running code and suspends execution when the current function returns. The debugger skips through the current function."](https://docs.microsoft.com/en-us/visualstudio/debugger/navigating-through-code-with-the-debugger?view=vs-2022#BKMK_Step_over_Step_out)
|
|
|
|
|
|
|
|
Then  Step Into:
|
|
|
|
|
|
|
|
Then  Step Out :
|
|
|
|
|
|
|
|
## Breakpoints
|
|
|
|
## Normal Breakpoints
|
|
|
|
**No breakpoint => No debugging.** You need to set breakpoints. Thus the debugger know where you want it to stop and allow you to investigate the state of your program.
|
|
|
|
You can create breakpoints by making these red dot in your source code. You just click left beside the line number (with the left mouse button) and get a new breakpoint.
|
|
|
|
|
|
|
|
If you click again then the red dot vanishes and the breakpoint is removed.
|
|
|
|
You can use the context menu (right mouse button) do deactivate or enable an existing breakpoint. In the case of a disabled breakpoint it turns gray.
|
|
|
|
|
|
|
|
In the breakpoint window, you see all your breakpoints. The X Symbol removes the breakpoint. Removing a blue checkmark disables the corresponding breakpoint.
|
|
|
|
|
|
|
|
### Expression
|
|
|
|
We can use the context menu (right mouse click) to add "Add Conditional Breakpoint...". Expression allow to define a condition that needs to be true to activate the breakpoint.
|
|
|
|
|
|
|
|
Now is a not fully filled dot.
|
|
|
|
|
|
|
|
You can use the context menu to change the condition or delete the breakpoint.
|
|
|
|
If you start debugging, this is where the debugger stops as initial breakpoint:
|
|
|
|
|
|
|
|
### Hit Count
|
|
|
|
Hit Count (defined via the context menu) allows to set an integer value for how often a position in a source code needs to be "hit" (i.e. run) until a breakpoint is activated.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
If you start debugging, this is where the debugger stops as initial breakpoint:
|
|
|
|
|
|
|
|
### Log Message / Add Log Point...
|
|
|
|
Log Message allows you to add the generation debug messages (messages in the debug console). If this "breakpoint" is passed by the debugger then a debug message is generated. However this is not a breakpoint that stops running the program like a normal breakpoint would. The message is defined akin to ["Formatted String Literals"](https://docs.python.org/3/tutorial/inputoutput.html#tut-f-strings) (also called f-string).
|
|
|
|
|
|
|
|
|
|
|
|
Running the debugger creates the following messages in the Debug Console:
|
|
|
|
|
|
|
|
### Function Breakpoints
|
|
|
|
In the Breakpoints window we can add Function Breakpoints. If the function named in the list is called that a breakpoint is initiated.
|
|
|
|
Use the + symbol:
|
|
|
|
|
|
|
|
|
|
|
|
### Raise Exception Breakpoints
|
|
|
|
You can break on throwing exceptions even if we catch them. However, we need another breakpoint (disabled or enabled) otherwise we cannot enable the Raise Exception breakpoints.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
## Starting the debug process
|
|
|
|
When we are happy with the placement of the breakpoints, we click on "Run and Debug". Then we tell the debugger that this is a Python file.
|
|
|
|
|
|
|
|
The yellow thingy tells us where in the source code we are in the moment:
|
|
|
|
|
|
|
|
If we press **Continue** then the debugger goes to the next breakpoint and we can view the content of the variables:
|
|
|
|
|
|
|
|
## Watch window
|
|
|
|
You can add variables to the watch window. This allows to observe variables and we can even watch slices of a numpy array:
|
|
|
|
|
|
|
|
You can add variables from the variable window via the context menu.
|
|
|
|
## Debug console
|
|
|
|
When the debugger waits for the next command from us, we can use the **Debug Console** to interact with the program. For example: We can print the value of variables or replace them with new values. If the console is not open already then you find it in the menu under View -> Debug Console.
|
|
|
|
|
|
|
|
|
|
## References
|
|
|
|
* [Navigate through code by using the Visual Studio debugger](https://docs.microsoft.com/en-us/visualstudio/debugger/navigating-through-code-with-the-debugger?view=vs-2022)
|
|
* [Debugging in VS Code](https://code.visualstudio.com/docs/editor/debugging)
|
|
* [Python debugging in VS Code](https://code.visualstudio.com/docs/python/debugging)
|