PDB Cheat Sheet

PDB, or Python Debugger, is a powerful tool that allows developers to debug their Python code. It is a command-line tool that provides a way to step through code, set breakpoints, and inspect variables and objects at runtime.

PDB is built into Python, so there is no need to install any additional software. To use PDB, simply add the following line of code to your Python script:

import pdb; pdb.set_trace()

This will pause the execution of your code at the point where the `set_trace()` function is called, allowing you to interactively debug your code.

Once you have entered the PDB debugger, you can use a variety of commands to navigate through your code. Some of the most commonly used commands include:

• `n` (next): Execute the current line and move to the next line.
• `s` (step): Step into a function call.
• `c` (continue): Continue execution until the next breakpoint is reached.
• `p` (print): Print the value of a variable or expression.
• `q` (quit): Quit the debugger.

PDB also provides a number of advanced features, such as the ability to set conditional breakpoints, examine the call stack, and interact with the Python interpreter.

Overall, PDB is an essential tool for any Python developer. By using PDB to debug your code, you can quickly identify and fix errors, saving time and improving the quality of your code.

This cheat sheet provides an overview of the most commonly used commands and features of the Python Debugger (PDB).

Starting PDB

Command	Description
python -m pdb script.py	Start PDB and run script.py
import pdb; pdb.set_trace()	Insert a breakpoint in your code

Basic Commands

Command	Description
h or help	Show a list of available commands
q or quit	Quit PDB
n or next	Execute the current line and move to the next line
s or step	Execute the current line and step into any function calls
c or continue	Continue execution until the next breakpoint
r or return	Continue execution until the current function returns
l or list	Show the current line and surrounding lines of code
w or where	Show the current stack trace
p or print	Evaluate and print an expression
pp or pprint	Evaluate and pretty-print an expression
a or args	Show the arguments of the current function
u or up	Move up the stack trace
d or down	Move down the stack trace

Breakpoints

Command	Description
b or break	Set a breakpoint at the current line
b file.py:line	Set a breakpoint at file.py:line
b function	Set a breakpoint at the start of function
b function:line	Set a breakpoint at function:line
b with no arguments	List all breakpoints
cl or clear	Clear a breakpoint
cl file.py:line	Clear the breakpoint at file.py:line
cl bpnumber	Clear the breakpoint with number bpnumber

Watchpoints

Command	Description
watch expression	Set a watchpoint on expression
watch -r expression	Set a reverse watchpoint on expression
watch with no arguments	List all watchpoints
rwatch expression	Set a watchpoint on expression that only triggers when it is read
rwatch -r expression	Set a reverse watchpoint on expression that only triggers when it is read
rwatch with no arguments	List all reverse watchpoints
awatch expression	Set an automatic watchpoint on expression that is deleted after it triggers
awatch -r expression	Set a reverse automatic watchpoint on expression that is deleted after it triggers
awatch with no arguments	List all automatic watchpoints

Stepping

Command	Description
s or step	Step into the current line
n or next	Step over the current line
unt or until	Continue execution until the current function returns or reaches a line greater than the current line
j or jump	Jump to a different line in the current function

Stack Trace

Command	Description
w or where	Show the current stack trace
u or up	Move up the stack trace
d or down	Move down the stack trace
bt or backtrace	Show the full stack trace

Variables

Command	Description
p or print	Print the value of a variable or expression
pp or pprint	Pretty-print the value of a variable or expression
a or args	Show the arguments of the current function
l or locals	Show the local variables of the current function
g or globals	Show the global variables
whatis expression	Show the type of expression
source expression	Show the source code of expression
interact	Start an interactive interpreter with access to the current frame

Miscellaneous

Command	Description
h or help	Show a list of available commands
!	Execute a Python statement in the current context
alias	Define an alias for a command
unalias	Remove an alias
pdbrc	Load a configuration file
run	Restart the program from the beginning

References

• Python documentation on PDB