Making the program stop at certain points
A breakpoint can make your program stop at that point. You can set breakpoints with the break command and its variants. You can specify the place where your program should stop by file and line number or by function name.
The debugger assigns a number to each breakpoint when you create it; these numbers are successive integers starting with 1. In many of the commands for controlling various features of breakpoints you use this number. Each breakpoint may be enabled or disabled; if disabled, it has no effect on your program until you enable it again.
The debugger allows you to set any number of breakpoints at the same place in your program. There is nothing unusual about this because different breakpoints can have different conditions associated with them.
The simplest sort of breakpoint breaks every time your program reaches a specified place. You can also specify a condition for a breakpoint. A condition is just a Boolean expression in your programming language. A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is true.
This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated-that is, when the condition is false.
Break conditions can have side effects, and may even call functions in your program. This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address. (In that case, pydb might see the other breakpoint first and stop your program without checking the condition of this one.) Note that breakpoint commands are usually more convenient and flexible than break conditions for the purpose of performing side effects when a breakpoint is reached.
Break conditions can be specified when a breakpoint is set, by adding a comma in the arguments to the break command. They can also be changed at any time with the condition command.