How to empty DBM file in Python efficiently?

Question

There is a command in DBM module to delete the value stored at a key.

del d[key]      # delete data stored at key (raises KeyError   # if no such key)

But I cannot even iterate with this command, because the Runtime error occurs.(RuntimeError: dictionary changed size during iteration.)

import dbm
db=dbm.open("file.db","c")
for key in db:
    del db[key]
print(len(db))
db.close()

Is there an efficient way to empty DMB file at once? I am using Python 3.3

Armin Rigo · Accepted Answer · 2013-05-21 07:48:17Z

3

for key in list(db):
    del db[key]

should work.

EDIT: If the goal is just to empty the database completely, you can also close the database and re-open it with dbm.open('filename', 'n'). The 'n' flag means "Always create a new, empty database, open for reading and writing"; it seems to override any previously-existing file.

edited May 21, 2013 at 7:48

answered May 20, 2013 at 12:41

Armin Rigo

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5 Comments

Sashko Kopyl Over a year ago

Thank you it works! Are there any solutions without iteration? Deleting files with OS.REMOVE is also an option, but DMB created 2 files. Aren't there a smart way like a DBM.CLEAR() or something?

Sashko Kopyl Over a year ago

I have tried it with "n" flag, but it does not override the file. >>> db=dbm.open("db","c") >>> db["line"]=("somedata") >>> db.close() >>> db=dbm.open("db","n") >>> data=db["line"] >>> print(data) b'somedata' >>> db.close()

Armin Rigo Over a year ago

Strange, it does for me (Python 2.7.3 on 32-bit Linux).

Sashko Kopyl Over a year ago

Maybe it is a Python version problem. I am using Python 3.3.2 32-bit on Windows7. If flag "n" does not override the file, there is no need for it, because a new file can be created with "c" flag.

Armin Rigo Over a year ago

Ah, found it. It's really buried in the docs :-( On Windows, it will use this: docs.python.org/3/library/dbm.html#module-dbm.dumb which documents its flag as ignored. I'd still file a bug report if I were you, saying that either the docs at docs.python.org/3/library/dbm.html#dbm.open should include a warning, or dumbdbm should be fixed.

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