cpython/Lib/dbm/__init__.py at 3.14 · python/cpython · GitHub

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"""Generic interface to all dbm clones.

Use

import dbm

d = dbm.open(file, 'w', 0o666)

The returned object is a dbm.sqlite3, dbm.gnu, dbm.ndbm or dbm.dumb database object, dependent on the

type of database being opened (determined by the whichdb function) in the case

of an existing dbm. If the dbm does not exist and the create or new flag ('c'

or 'n') was specified, the dbm type will be determined by the availability of

the modules (tested in the above order).

It has the following interface (key and data are strings):

d[key] = data # store data at key (may override data at

# existing key)

data = d[key] # retrieve data at key (raise KeyError if no

# such key)

del d[key] # delete data stored at key (raises KeyError

# if no such key)

flag = key in d # true if the key exists

list = d.keys() # return a list of all existing keys (slow!)

Future versions may change the order in which implementations are

tested for existence, and add interfaces to other dbm-like

implementations.

"""

__all__ = ['open', 'whichdb', 'error']

import io

import os

import struct

import sys

class error(Exception):

pass

_names = ['dbm.sqlite3', 'dbm.gnu', 'dbm.ndbm', 'dbm.dumb']

_defaultmod = None

_modules = {}

error = (error, OSError)

try:

from dbm import ndbm

except ImportError:

ndbm = None

def open(file, flag='r', mode=0o666):

"""Open or create database at path given by *file*.

Optional argument *flag* can be 'r' (default) for read-only access, 'w'

for read-write access of an existing database, 'c' for read-write access

to a new or existing database, and 'n' for read-write access to a new

database.

Note: 'r' and 'w' fail if the database doesn't exist; 'c' creates it

only if it doesn't exist; and 'n' always creates a new database.

"""

global _defaultmod

if _defaultmod is None:

for name in _names:

try:

mod = __import__(name, fromlist=['open'])

except ImportError:

continue

if not _defaultmod:

_defaultmod = mod

_modules[name] = mod

if not _defaultmod:

raise ImportError("no dbm clone found; tried %s" % _names)

# guess the type of an existing database, if not creating a new one

result = whichdb(file) if 'n' not in flag else None

if result is None:

# db doesn't exist or 'n' flag was specified to create a new db

if 'c' in flag or 'n' in flag:

# file doesn't exist and the new flag was used so use default type

mod = _defaultmod

else:

raise error[0]("db file doesn't exist; "

"use 'c' or 'n' flag to create a new db")

elif result == "":

# db type cannot be determined

raise error[0]("db type could not be determined")

elif result not in _modules:

raise error[0]("db type is {0}, but the module is not "

"available".format(result))

else:

mod = _modules[result]

return mod.open(file, flag, mode)

def whichdb(filename):

"""Guess which db package to use to open a db file.

Return values:

- None if the database file can't be read;

- empty string if the file can be read but can't be recognized

- the name of the dbm submodule (e.g. "ndbm" or "gnu") if recognized.

Importing the given module may still fail, and opening the

database using that module may still fail.

"""

# Check for ndbm first -- this has a .pag and a .dir file

filename = os.fsencode(filename)

try:

f = io.open(filename + b".pag", "rb")

f.close()

f = io.open(filename + b".dir", "rb")

f.close()

return "dbm.ndbm"

except OSError:

# some dbm emulations based on Berkeley DB generate a .db file

# some do not, but they should be caught by the bsd checks

try:

f = io.open(filename + b".db", "rb")

f.close()

# guarantee we can actually open the file using dbm

# kind of overkill, but since we are dealing with emulations

# it seems like a prudent step

if ndbm is not None:

d = ndbm.open(filename)

d.close()

return "dbm.ndbm"

except OSError:

pass

# Check for dumbdbm next -- this has a .dir and a .dat file

try:

# First check for presence of files

os.stat(filename + b".dat")

size = os.stat(filename + b".dir").st_size

# dumbdbm files with no keys are empty

if size == 0:

return "dbm.dumb"

f = io.open(filename + b".dir", "rb")

try:

if f.read(1) in (b"'", b'"'):

return "dbm.dumb"

finally:

f.close()

except OSError:

pass

# See if the file exists, return None if not

try:

f = io.open(filename, "rb")

except OSError:

return None

with f:

# Read the start of the file -- the magic number

s16 = f.read(16)

s = s16[0:4]

# Return "" if not at least 4 bytes

if len(s) != 4:

return ""

# Check for SQLite3 header string.

if s16 == b"SQLite format 3\0":

return "dbm.sqlite3"

# Convert to 4-byte int in native byte order -- return "" if impossible

try:

(magic,) = struct.unpack("=l", s)

except struct.error:

return ""

# Check for GNU dbm

if magic in (0x13579ace, 0x13579acd, 0x13579acf):

return "dbm.gnu"

# Later versions of Berkeley db hash file have a 12-byte pad in

# front of the file type

try:

(magic,) = struct.unpack("=l", s16[-4:])

except struct.error:

return ""

# Unknown

return ""

if __name__ == "__main__":

for filename in sys.argv[1:]:

print(whichdb(filename) or "UNKNOWN", filename)

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