Using a debugger

The main features of a debugger are:

• Breakpoints: pause the program when a particular line of code is about to be executed;

• Display/print: show the current value of local variables;

• Next: execute the current line of code and pause at the next line;

• Continue: continue executing code until the next breakpoint, or the code finishes.

Slightly more advanced features include:

• Conditional breakpoints: pause the program when a particular line of code is about to be executed and a specific condition is satisfied.

• Step: execute the current line of code and pause at the first possible point — either the line in the current function or the first line in a function that is called.

• Manual breakpoints: call breakpoint() in Python, and browser() in R, to pause the program at the current line.

For example, consider the following code example:

PythonR

def first_function():
    total = 0
    for x in range(1, 50):
        y = second_function(x)
        total = total + y

    return total


def second_function(a):
    result = 3 * a**2 + 5 * a
    return result


total = first_function()
print(f'Total = {total}')

first_function <- function() {
  total <- 0
  for (x in seq(49)) {
    y <- second_function(x)
    total <- total + y
  }
  total
}

second_function <- function(a) {
  result <- 3 * a^2 + 5 * a
  result
}

total <- first_function()
cat("Total =", total, "\n")

• We can use a conditional breakpoint to pause on line 4 (highlighted) only when x = 42.

• We can then use step to begin executing line 4 and pause on line 11, where we will see that a = 42.

• If we instead used next at line 4 (highlighted), the debugger would execute line 4 and then pause on line 5.

Try this yourself

Download either demo script and try setting breakpoints and stepping through the code.

• first_demo.py
• first_demo.R

Interactive debugger sessions

If your editor supports running a debugger, use this feature! See these examples for RStudio, PyCharm, Spyder, and VS Code.