How to use pdb to debug python in Linux environment

pdb is a package that comes with python. It provides an interactive source code debugging function for python programs. Its main features include setting breakpoints, single-step debugging, entering function debugging, viewing current code, viewing stack fragments, and dynamic Change the value of a variable, etc. pdb provides some commonly used debugging commands.

Description of common commands for pdb:

l You can enter the function
p variable #View variable value
b line number #break breakpoint is set to which line
b #break displays a list of all breakpoints
cl breakpoint number #clear delete a certain Breakpoints
Cl
　pdb setting breakpoints can be added to the program:
　import pdb
　Add pdb.set_trace() where you need to set breakpoints
　　Execute python -m pdb test.py

The following describes how to use pdb for debugging with specific examples.


List 1. Test code example
import pdb
a = "aaa"
pdb.set_trace()
b = "bbb"
c = " ccc"
final = a + b + c
print final

List 2. Debugging using pdb
[root@rcc-pok-idg-2255 ~]# python epdb1.py
> /root/epdb1.py(4)?()
-> b = "bbb"
(Pdb) n
> /root/epdb1.py(5)?( )
-> c = "ccc"
(Pdb)
> /root/epdb1.py(6)?()
-> final = a + b + c
(Pdb) list
1 import pdb
2 a = "aaa"
3 pdb.set_trace()
4 b = "bbb"
5 c = "ccc"
6 -> final = a + b + c
7 print final
[EOF]
(Pdb)
[EOF]
(Pdb) n
> /root/ epdb1.py(7)?()
-> print final
(Pdb)

Print the value of the variable: If you need to print the value of the variable during debugging, you can directly use p plus above the variable name, but it should be noted that printing can only see the specific value after the current statement has been executed, otherwise a NameError: < exceptions.NameError … ....> error will be reported.

List 3. Print variables during debug process
[root@rcc-pok-idg-2255 ~]# python epdb1.py
> /root/epdb1.py(4)?( )
-> b = "bbb"
(Pdb) n
> /root/epdb1.py(5)?()
-> c = "ccc"
(Pdb) p b
'bbb'
(Pdb)
'bbb'
(Pdb) n
> /root/epdb1.py(6)?()
- > final = a + b + c
(Pdb) p c
'ccc'
(Pdb) p final
*** NameError:
(Pdb) n
> /root/epdb1.py(7)?()
-> print final
(Pdb) p final
'aaabbbccc'
(Pdb)

Use c to stop the current debug and allow the program to continue executing. If there are further statements of set_statement() in the following program, it will re-enter the debug state. Readers can add set_trace() verification before the code print final.


List 4. Stop debugging and continue executing the program
[root@rcc-pok-idg-2255 ~]# python epdb1.py
> /root/epdb1.py(4 )?()
-> b = "bbb"
(Pdb) n
> /root/epdb1.py(5)?()
-> c = "ccc"
(Pdb) c
aaabbbccc

Display code: The current code block may not be remembered during debugging. If you want to view a specific code block, you can display it by using the list or l command. list will use the arrow -> to point to the current debug statement.

List 6. Code displayed during debug process
[root@rcc-pok-idg-2255 ~]# python epdb1.py
> /root/epdb1.py(4)?( )
-> b = "bbb"
(Pdb) list
1 import pdb
2 a = "aaa"
3 pdb.set_trace()
4 -> b = "bbb"
5 c = "ccc"
6 final = a + b + c
7 pdb.set_trace()
8 print final
[EOF]
( Pdb) c
> /root/epdb1.py(8)?()
-> print final
(Pdb) list
3 pdb.set_trace()
4 b = "bbb"
5 c = "ccc"
6 final = a + b + c
7 pdb.set_trace()
8 -> print final
[EOF]
(Pdb)


Debug when using functions


Listing 6. Example of using functions
import pdb
def combine(s1,s2 ): # define subroutine combine, which...
s3 = s1 + s2 + s1 # sandwiches s2 between copies of s1,...
s3 = '"' + s3 +'"' # encloses it in double quotes,...
return s3 # and returns it.
a = "aaa"
pdb.set_trace()
b = "bbb"
c = "ccc"
final = combine(a,b)
print final

If you use n directly for debugging, it will be treated as an ordinary assignment when you reach the final=combine(a,b) sentence. Statement processing, enter print final. What if you want to debug a function? You can directly use s to enter the function block. Single-step debugging in the function is similar to the above introduction. If you don’t want to step through the function, you can press r directly at the breakpoint to exit to the calling place.

清单 7. 对函数进行 debug
[root@rcc-pok-idg-2255 ~]# python epdb2.py 
 > /root/epdb2.py(10)?() 
 -> b = "bbb"
 (Pdb) n 
 > /root/epdb2.py(11)?() 
 -> c = "ccc"
 (Pdb) n 
 > /root/epdb2.py(12)?() 
 -> final = combine(a,b) 
 (Pdb) s 
 --Call-- 
 > /root/epdb2.py(3)combine() 
 -> def combine(s1,s2):      # define subroutine combine, which... 
 (Pdb) n 
 > /root/epdb2.py(4)combine() 
 -> s3 = s1 + s2 + s1    # sandwiches s2 between copies of s1, ... 
 (Pdb) list 
  1     import pdb 
  2 
  3     def combine(s1,s2):      # define subroutine combine, which... 
  4  ->     s3 = s1 + s2 + s1    # sandwiches s2 between copies of s1, ... 
  5         s3 = '"' + s3 +'"'   # encloses it in double quotes,... 
  6         return s3            # and returns it. 
  7 
  8     a = "aaa"
  9     pdb.set_trace() 
 10     b = "bbb"
 11     c = "ccc"
 (Pdb) n 
 > /root/epdb2.py(5)combine() 
 -> s3 = '"' + s3 +'"'   # encloses it in double quotes,... 
 (Pdb) n 
 > /root/epdb2.py(6)combine() 
 -> return s3            # and returns it. 
 (Pdb) n 
 --Return-- 
 > /root/epdb2.py(6)combine()->'"aaabbbaaa"'
 -> return s3            # and returns it. 
 (Pdb) n 
 > /root/epdb2.py(13)?() 
 -> print final 
 (Pdb)


在调试的时候动态改变值 。在调试的时候可以动态改变变量的值，具体如下实例。需要注意的是下面有个错误，原因是 b 已经被赋值了，如果想重新改变 b 的赋值，则应该使用!b。


清单 8. 在调试的时候动态改变值
[root@rcc-pok-idg-2255 ~]# python epdb2.py 

> /root/epdb2.py(10)?() 
 -> b = "bbb"
 (Pdb) var = "1234"
 (Pdb) b = "avfe"
 *** The specified object '= "avfe"' is not a function 
 or was not found along sys.path. 
 (Pdb) !b="afdfd"
 (Pdb)

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