[Ipdb] Web development beginners tried to summarize debugging with Python

Introduction

I will summarize debugging using ipdb, including organizing what the web development beginners have learned.

What is ipdb?

It provides an extension of pdb, which is a ** debugger ** that comes standard with Python.

How to use

Enter the following code at the position where you want to start the debugger and execute the program.

import ipdb; ipdb.set_trace()

When executed, it waits for ** standard input **, so you can operate the debugger by entering a command. The following is a summary of frequently used (likely) commands.

Try using

Let's actually use ipdb using the program of Aggressive Cows of POJ No.3468.

Aggressive.py

import ipdb
n = 5
m = 3
x = [1, 2, 8, 4, 9]

def C(d):
    last = 0
    for _ in range(1, m):
        crt = last + 1
        while crt<n and x[crt]-x[last]<d:
            crt += 1
        if crt == n:
            return False
        last = crt
    return True

x.sort()
lb, ub = 0, max(x)

while ub-lb > 1:
    ipdb.set_trace()
    mid = int( (lb+ub)/2 )
    if C(mid):
        lb = mid
    else:
        ub = mid

print(lb)

Execute.

$ python Aggressive.py
>c:/users/~/aggressive.py(21)<module>()
     20     import ipdb; ipdb.set_trace()
---> 21     mid = int( (lb+ub)/2 )
     22     if C(mid):

The set breakpoint is output as an arrow.

ipdb> h

Documented commands (type help <topic>):
========================================
EOF    cl         disable  interact  next    psource  rv         unt
a      clear      display  j         p       q        s          until
alias  commands   down     jump      pdef    quit     source     up
args   condition  enable   l         pdoc    r        step       w
b      cont       exit     list      pfile   restart  tbreak     whatis
break  continue   h        ll        pinfo   return   u          where
bt     d          help     longlist  pinfo2  retval   unalias
c      debug      ignore   n         pp      run      undisplay

Miscellaneous help topics:
==========================
exec  pdb

A list of usable commands is output.

ipdb> p mid
*** NameError: name 'mid' is not defined

The variable mid is not defined because line 21 has not been executed yet.

ipdb> n
> c:/users/~/aggressive.py(22)<module>()
     21     mid = int( (lb+ub)/2 )
---> 22     if C(mid):
     23         lb = mid

The arrow indicating the breakpoint shifts down by one.

ipdb> p mid
4

Since the 21st line is executed, the value of the variable mid is displayed.

ipdb> s
--Call--
> c:/users/~/aggressive.py(6)C()
      5 
----> 6 def C(d):
      7     last = 0

It seems that the call to the next function was executed.

ipdb> a
d = 4

Since the call to the next function was just below, the value of the variable mid confirmed earlier is displayed as an argument.

ipdb> q
Exiting Debugger.

Exit the debugger.

Summary

I used all the commands that are often used in ipdb. It was my first time to use the debugger, but I ran the program ** halfway ** and checked the contents of the variables each time, or ran it to the desired position without rewriting the program. I found it very convenient to go. It's hard to write a program because it's debugging, so I want to make good use of the debugger to write an efficient and error-free program.