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Getting started with the Python debugger
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======
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The Python ecosystem is rich with many tools and libraries that improve developers’ lives. For example, the Magazine has previously covered how to [enhance your Python with a interactive shell][1]. This article focuses on another tool that saves you time and improves your Python skills: the Python debugger.
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### Python Debugger
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The Python standard library provides a debugger called pdb. This debugger provides most features needed for debugging such as breakpoints, single line stepping, inspection of stack frames, and so on.
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A basic knowledge of pdb is useful since it’s part of the standard library. You can use it in environments where you can’t install another enhanced debugger.
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#### Running pdb
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The easiest way to run pdb is from the command line, passing the program to debug as an argument. Considering the following script:
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```
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# pdb_test.py
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#!/usr/bin/python3
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from time import sleep
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def countdown(number):
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for i in range(number, 0, -1):
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print(i)
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sleep(1)
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if __name__ == "__main__":
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seconds = 10
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countdown(seconds)
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```
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You can run pdb from the command line like this:
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```
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$ python3 -m pdb pdb_test.py
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> /tmp/pdb_test.py(1)<module>()
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-> from time import sleep
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(Pdb)
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```
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Another way to use pdb is to set a breakpoint in the program. To do this, import the pdb module and use the set_trace function:
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```
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1 # pdb_test.py
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2 #!/usr/bin/python3
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3
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4 from time import sleep
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5
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6
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7 def countdown(number):
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8 for i in range(number, 0, -1):
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9 import pdb; pdb.set_trace()
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10 print(i)
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11 sleep(1)
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12
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13
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14 if __name__ == "__main__":
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15 seconds = 10
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16 countdown(seconds)
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$ python3 pdb_test.py
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> /tmp/pdb_test.py(6)countdown()
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-> print(i)
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(Pdb)
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```
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The script stops at the breakpoint, and pdb displays the next line in the script. You can also execute the debugger after a failure. This is known as postmortem debugging.
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#### Navigate the execution stack
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A common use case in debugging is to navigate the execution stack. Once the Python debugger is running, the following commands are useful :
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+ w(here) : Shows which line is currently executed and where the execution stack is.
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```
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$ python3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) w
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/tmp/test_pdb.py(16)<module>()
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-> countdown(seconds)
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb)
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```
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+ l(ist) : Shows more context (code) around the current the location.
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```
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$ python3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) l
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5
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6
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7 def countdown(number):
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8 for i in range(number, 0, -1):
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9 import pdb; pdb.set_trace()
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10 -> print(i)
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11 sleep(1)
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12
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13
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14 if __name__ == "__main__":
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15 seconds = 10
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(Pdb)
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```
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+ u(p)/d(own) : Navigate the call stack up or down.
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```
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$ py3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) up
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> /tmp/test_pdb.py(16)<module>()
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-> countdown(seconds)
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(Pdb) down
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb)
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```
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#### Stepping through a program
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pdb provides the following commands to execute and step through code:
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+ n(ext): Continue execution until the next line in the current function is reached, or it returns
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+ s(tep): Execute the current line and stop at the first possible occasion (either in a function that is called or in the current function)
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+ c(ontinue): Continue execution, only stopping at a breakpoint.
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```
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$ py3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) n
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10
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> /tmp/test_pdb.py(11)countdown()
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-> sleep(1)
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(Pdb) n
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> /tmp/test_pdb.py(8)countdown()
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-> for i in range(number, 0, -1):
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(Pdb) n
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> /tmp/test_pdb.py(9)countdown()
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-> import pdb; pdb.set_trace()
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(Pdb) s
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--Call--
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> /usr/lib64/python3.6/pdb.py(1584)set_trace()
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-> def set_trace():
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(Pdb) c
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) c
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9
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> /tmp/test_pdb.py(9)countdown()
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-> import pdb; pdb.set_trace()
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(Pdb)
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```
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The example shows the difference between next and step. Indeed, when using step the debugger stepped into the pdb module source code, whereas next would have just executed the set_trace function.
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#### Examine variables content
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Where pdb is really useful is examining the content of variables stored in the execution stack. For example, the a(rgs) command prints the variables of the current function, as shown below:
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```
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py3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) where
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/tmp/test_pdb.py(16)<module>()
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-> countdown(seconds)
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) args
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number = 10
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(Pdb)
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```
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pdb prints the value of the variable number, in this case 10.
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Another command that can be used to print variables value is p(rint).
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```
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$ py3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) list
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5
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6
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7 def countdown(number):
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8 for i in range(number, 0, -1):
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9 import pdb; pdb.set_trace()
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10 -> print(i)
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11 sleep(1)
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12
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13
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14 if __name__ == "__main__":
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15 seconds = 10
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(Pdb) print(seconds)
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10
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(Pdb) p i
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10
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(Pdb) p number - i
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0
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(Pdb)
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```
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As shown in the example’s last command, print can evaluate an expression before displaying the result.
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The [Python documentation][2] contains the reference and examples for each of the pdb commands. This is a useful read for someone starting with the Python debugger.
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### Enhanced debugger
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Some enhanced debuggers provide a better user experience. Most add useful extra features to pdb, such as syntax highlighting, better tracebacks, and introspection. Popular choices of enhanced debuggers include [IPython’s ipdb][3] and [pdb++][4].
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These examples show you how to install these two debuggers in a virtual environment. These examples use a new virtual environment, but in the case of debugging an application, the application’s virtual environment should be used.
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#### Install IPython’s ipdb
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To install the IPython ipdb, use pip in the virtual environment:
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```
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$ python3 -m venv .test_pdb
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$ source .test_pdb/bin/activate
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(test_pdb)$ pip install ipdb
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```
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To call ipdb inside a script, you must use the following command. Note that the module is called ipdb instead of pdb:
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```
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import ipdb; ipdb.set_trace()
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```
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IPython’s ipdb is also available in Fedora packages, so you can install it using Fedora’s package manager dnf:
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```
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$ sudo dnf install python3-ipdb
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```
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#### Install pdb++
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You can install pdb++ similarly:
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```
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$ python3 -m venv .test_pdb
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$ source .test_pdb/bin/activate
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(test_pdb)$ pip install pdbp
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```
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pdb++ overrides the pdb module, and therefore you can use the same syntax to add a breakpoint inside a program:
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```
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import pdb; pdb.set_trace()
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```
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### Conclusion
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Learning how to use the Python debugger saves you time when investigating problems with an application. It can also be useful to understand how a complex part of an application or some libraries work, and thereby improve your Python developer skills.
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--------------------------------------------------------------------------------
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via: https://fedoramagazine.org/getting-started-python-debugger/
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作者：[Clément Verna][a]
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选题：[lujun9972](https://github.com/lujun9972)
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://fedoramagazine.org
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[1]:https://fedoramagazine.org/enhance-python-interactive-shell
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[2]:https://docs.python.org/3/library/pdb.html
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[3]:https://github.com/gotcha/ipdb
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[4]:https://github.com/antocuni/pdb
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@ -0,0 +1,281 @@
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开始使用Python调试器
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======
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Python生态系统包含丰富的工具和库，可以改善开发人员的生活。 例如，杂志之前已经介绍了如何[使用交互式shell增强Python][1]。 本文重点介绍另一种可以节省时间并提高Python技能的工具：Python调试器。
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### Python调试器
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Python标准库提供了一个名为pdb的调试器。 此调试器提供了调试所需的大多数功能，如断点，单行步进，堆栈帧的检查等等。
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pdb的基本知识很有用，因为它是标准库的一部分。 你可以在无法安装其他增强的调试器的环境中使用它。
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#### 运行pdb
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运行pdb的最简单方法是从命令行，将程序作为参数传递给debug。 考虑以下脚本：
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```
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# pdb_test.py
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#!/usr/bin/python3
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from time import sleep
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def countdown(number):
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for i in range(number, 0, -1):
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print(i)
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sleep(1)
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if __name__ == "__main__":
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seconds = 10
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countdown(seconds)
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```
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你可以从命令行运行pdb，如下所示：
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```
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$ python3 -m pdb pdb_test.py
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> /tmp/pdb_test.py(1)<module>()
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-> from time import sleep
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(Pdb)
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```
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使用pdb的另一种方法是在程序中设置断点。 为此，请导入pdb模块并使用set_trace函数：
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```
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# pdb_test.py
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#!/usr/bin/python3
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from time import sleep
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def countdown(number):
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for i in range(number, 0, -1):
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import pdb; pdb.set_trace()
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print(i)
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sleep(1)
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if __name__ == "__main__":
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seconds = 10
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countdown(seconds)
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$ python3 pdb_test.py
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> /tmp/pdb_test.py(6)countdown()
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-> print(i)
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(Pdb)
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```
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脚本在断点处停止，pdb显示脚本中的下一行。 你也可以在失败后执行调试器。 这称为*事后调试（postmortem debugging）*。
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#### 导航执行堆栈
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调试中的一个常见用例是导航执行堆栈。 Python调试器运行后，以下命令很有用：
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+ w(here) : 显示当前执行的行以及执行堆栈的位置。
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```
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$ python3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) w
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/tmp/test_pdb.py(16)<module>()
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-> countdown(seconds)
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb)
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```
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+ l(ist) : 显示当前位置周围更多的上下文（代码）。
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```
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$ python3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) l
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5
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6
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7 def countdown(number):
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8 for i in range(number, 0, -1):
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9 import pdb; pdb.set_trace()
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10 -> print(i)
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11 sleep(1)
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12
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13
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14 if __name__ == "__main__":
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15 seconds = 10
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```
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+ u(p)/d(own) : 向上或向下导航调用堆栈。
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```
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$ py3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) up
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> /tmp/test_pdb.py(16)<module>()
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-> countdown(seconds)
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(Pdb) down
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb)
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```
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#### 单步执行程序
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pdb提供以下命令来执行和单步执行代码：
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+ n(ext): 继续执行，直到达到当前函数中的下一行，否则返回
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+ s(tep): 执行当前行并在第一个可能的场合停止（在被调用的函数或当前函数中）
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+ c(ontinue): 继续执行，仅在断点处停止。
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|
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```
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$ py3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) n
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10
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> /tmp/test_pdb.py(11)countdown()
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-> sleep(1)
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(Pdb) n
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> /tmp/test_pdb.py(8)countdown()
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-> for i in range(number, 0, -1):
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(Pdb) n
|
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> /tmp/test_pdb.py(9)countdown()
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-> import pdb; pdb.set_trace()
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(Pdb) s
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--Call--
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> /usr/lib64/python3.6/pdb.py(1584)set_trace()
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-> def set_trace():
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(Pdb) c
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) c
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9
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> /tmp/test_pdb.py(9)countdown()
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-> import pdb; pdb.set_trace()
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(Pdb)
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```
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该示例显示了next和step之间的区别。 实际上，当使用step时，调试器会进入pdb模块源代码，而接下来就会执行set_trace函数。
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#### 检查变量内容
|
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pdb非常有用的地方是检查执行堆栈中存储的变量的内容。 例如，a(rgs)命令打印当前函数的变量，如下所示：
|
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|
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```
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py3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) where
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/tmp/test_pdb.py(16)<module>()
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-> countdown(seconds)
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) args
|
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number = 10
|
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(Pdb)
|
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```
|
||||
|
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pdb打印变量的值，在本例中是10。
|
||||
|
||||
可用于打印变量值的另一个命令是p(rint)。
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||||
|
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```
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||||
$ py3 test_pdb.py
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> /tmp/test_pdb.py(10)countdown()
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-> print(i)
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(Pdb) list
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5
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6
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7 def countdown(number):
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||||
8 for i in range(number, 0, -1):
|
||||
9 import pdb; pdb.set_trace()
|
||||
10 -> print(i)
|
||||
11 sleep(1)
|
||||
12
|
||||
13
|
||||
14 if __name__ == "__main__":
|
||||
15 seconds = 10
|
||||
(Pdb) print(seconds)
|
||||
10
|
||||
(Pdb) p i
|
||||
10
|
||||
(Pdb) p number - i
|
||||
0
|
||||
(Pdb)
|
||||
```
|
||||
|
||||
如示例中最后的命令所示，print可以在显示结果之前计算表达式。
|
||||
|
||||
[Python文档][2]包含每个pdb命令的参考和示例。 对于开始使用Python调试器人来说，这是一个有用的读物。
|
||||
|
||||
### 增强的调试器
|
||||
|
||||
一些增强的调试器提供了更好的用户体验。 大多数为pdb添加了有用的额外功能，例如语法突出高亮，更好的回溯和自我检查。 流行的增强调试器包括[IPython的ipdb][3]和[pdb ++][4]。
|
||||
|
||||
这些示例显示如何在虚拟环境中安装这两个调试器。 这些示例使用新的虚拟环境，但在调试应用程序的情况下，应使用应用程序的虚拟环境。
|
||||
|
||||
#### 安装IPython的ipdb
|
||||
|
||||
要安装IPython ipdb，请在虚拟环境中使用pip：
|
||||
|
||||
```
|
||||
$ python3 -m venv .test_pdb
|
||||
$ source .test_pdb/bin/activate
|
||||
(test_pdb)$ pip install ipdb
|
||||
```
|
||||
|
||||
要在脚本中调用ipdb，必须使用以下命令。 请注意，该模块称为ipdb而不是pdb：
|
||||
|
||||
```
|
||||
import ipdb; ipdb.set_trace()
|
||||
```
|
||||
|
||||
IPython的ipdb也可以在Fedora包中使用，所以你可以使用Fedora的包管理器dnf来安装它：
|
||||
|
||||
```
|
||||
$ sudo dnf install python3-ipdb
|
||||
```
|
||||
|
||||
#### 安装pdb++
|
||||
|
||||
你可以类似地安装pdb++：
|
||||
|
||||
```
|
||||
$ python3 -m venv .test_pdb
|
||||
$ source .test_pdb/bin/activate
|
||||
(test_pdb)$ pip install pdbp
|
||||
```
|
||||
|
||||
pdb++重写了pdb模块，因此你可以使用相同的语法在程序中添加断点：
|
||||
|
||||
```
|
||||
import pdb; pdb.set_trace()
|
||||
```
|
||||
|
||||
### Conclusion
|
||||
|
||||
学习如何使用Python调试器可以节省你在排查应用程序问题时的时间。 对于了解应用程序或某些库的复杂部分如何工作也是有用的，从而提高Python开发人员的技能。
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: https://fedoramagazine.org/getting-started-python-debugger/
|
||||
|
||||
作者：[Clément Verna][a]
|
||||
选题：[lujun9972](https://github.com/lujun9972)
|
||||
译者：[Flowsnow](https://github.com/Flowsnow)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]:https://fedoramagazine.org
|
||||
[1]:https://fedoramagazine.org/enhance-python-interactive-shell
|
||||
[2]:https://docs.python.org/3/library/pdb.html
|
||||
[3]:https://github.com/gotcha/ipdb
|
||||
[4]:https://github.com/antocuni/pdb
|
||||
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Reference in New Issue
Block a user