Debugging a Click application using pdb#

This tip is for when you are working on a Python command-line application that runs using that program’s name, as opposed to typing python my_script.py. I usually need this when I’m working on applications built using Click, e.g. projects I start using my click-app cookiecutter template.

So you’re working on one of these apps and it throws an exception:

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% dogsheep-beta index beta.db dogsheep-beta.yml
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Traceback (most recent call last):
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...
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  File "/Users/simon/Dropbox/Development/dogsheep-beta/dogsheep_beta/utils.py", line 46, in run_indexer
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    columns = derive_columns(other_db, sql)
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  File "/Users/simon/Dropbox/Development/dogsheep-beta/dogsheep_beta/utils.py", line 63, in derive_columns
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    cursor = db.conn.execute(sql + " limit 0")
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sqlite3.OperationalError: near "from": syntax error

You can run the Python debugger against it using python -i $(which dogsheep-beta) to get an interactive prompt, then import pdb; pdb.pm() to open the debugger at the last exception:

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% python -i $(which dogsheep-beta) index beta.db dogsheep-beta.yml
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Traceback (most recent call last):
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  File "/Users/simon/.local/share/virtualenvs/dogsheep-beta-u_po4Rpj/bin/dogsheep-beta", line 11, in <module>
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    load_entry_point('dogsheep-beta', 'console_scripts', 'dogsheep-beta')()
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  File "/Users/simon/.local/share/virtualenvs/dogsheep-beta-u_po4Rpj/lib/python3.8/site-packages/click/core.py", line 829, in __call__
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    return self.main(*args, **kwargs)
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  File "/Users/simon/.local/share/virtualenvs/dogsheep-beta-u_po4Rpj/lib/python3.8/site-packages/click/core.py", line 782, in main
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    rv = self.invoke(ctx)
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  File "/Users/simon/.local/share/virtualenvs/dogsheep-beta-u_po4Rpj/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
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    return _process_result(sub_ctx.command.invoke(sub_ctx))
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  File "/Users/simon/.local/share/virtualenvs/dogsheep-beta-u_po4Rpj/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
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    return ctx.invoke(self.callback, **ctx.params)
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  File "/Users/simon/.local/share/virtualenvs/dogsheep-beta-u_po4Rpj/lib/python3.8/site-packages/click/core.py", line 610, in invoke
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    return callback(*args, **kwargs)
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  File "/Users/simon/Dropbox/Development/dogsheep-beta/dogsheep_beta/cli.py", line 30, in index
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    run_indexer(db_path, rules, tokenize=None if tokenize == "none" else tokenize)
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  File "/Users/simon/Dropbox/Development/dogsheep-beta/dogsheep_beta/utils.py", line 46, in run_indexer
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    columns = derive_columns(other_db, sql)
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  File "/Users/simon/Dropbox/Development/dogsheep-beta/dogsheep_beta/utils.py", line 63, in derive_columns
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    cursor = db.conn.execute(sql + " limit 0")
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sqlite3.OperationalError: near "from": syntax error
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>>> import pdb; pdb.pm()
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> /Users/simon/Dropbox/Development/dogsheep-beta/dogsheep_beta/utils.py(63)derive_columns()
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-> cursor = db.conn.execute(sql + " limit 0")
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(Pdb) print(sql)
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...