dbm
dbm.gnu
dbm.ndbm
dbm.dumb
marshal — 内部 Python 对象序列化
marshal
源代码： Lib/dbm/__init__.py
dbm 是 DBM 数据库变体的一般接口 — dbm.gnu or dbm.ndbm . If none of these modules is installed, the slow-but-simple implementation in module dbm.dumb will be used. There is a third party interface to the Oracle Berkeley DB.
A tuple containing the exceptions that can be raised by each of the supported modules, with a unique exception also named dbm.error as the first item — the latter is used when dbm.error 被引发。
dbm.error
This function attempts to guess which of the several simple database modules available — dbm.gnu , dbm.ndbm or dbm.dumb — should be used to open a given file.
Return one of the following values:
None if the file can’t be opened because it’s unreadable or doesn’t exist
None
the empty string ( '' ) if the file’s format can’t be guessed
''
a string containing the required module name, such as 'dbm.ndbm' or 'dbm.gnu'
'dbm.ndbm'
'dbm.gnu'
3.11 版改变： filename accepts a 像路径对象 .
Open a database and return the corresponding database object.
file ( 像路径对象 ) –
The database file to open.
If the database file already exists, the whichdb() function is used to determine its type and the appropriate module is used; if it does not exist, the first submodule listed above that can be imported is used.
whichdb()
flag ( str ) –
'r' (default): Open existing database for reading only.
'r'
'w' : Open existing database for reading and writing.
'w'
'c' : Open database for reading and writing, creating it if it doesn’t exist.
'c'
'n' : Always create a new, empty database, open for reading and writing.
'n'
mode ( int ) – The Unix file access mode of the file (default: octal 0o666 ), used only when the database has to be created.
0o666
3.11 版改变： file accepts a 像路径对象 .
The object returned by open() supports the same basic functionality as a dict ; keys and their corresponding values can be stored, retrieved, and deleted, and the in operator and the keys() method are available, as well as get() and setdefault() 方法。
open()
dict
in
keys()
get()
setdefault()
Key and values are always stored as bytes . This means that when strings are used they are implicitly converted to the default encoding before being stored.
bytes
These objects also support being used in a with statement, which will automatically close them when done.
with
3.2 版改变： get() and setdefault() methods are now available for all dbm backends.
3.4 版改变： Added native support for the context management protocol to the objects returned by open() .
3.8 版改变： Deleting a key from a read-only database raises a database module specific exception instead of KeyError .
KeyError
The following example records some hostnames and a corresponding title, and then prints out the contents of the database:
import dbm # Open database, creating it if necessary. with dbm.open('cache', 'c') as db: # Record some values db[b'hello'] = b'there' db['www.python.org'] = 'Python Website' db['www.cnn.com'] = 'Cable News Network' # Note that the keys are considered bytes now. assert db[b'www.python.org'] == b'Python Website' # Notice how the value is now in bytes. assert db['www.cnn.com'] == b'Cable News Network' # Often-used methods of the dict interface work too. print(db.get('python.org', b'not present')) # Storing a non-string key or value will raise an exception (most # likely a TypeError). db['www.yahoo.com'] = 4 # db is automatically closed when leaving the with statement.
另请参阅
shelve
Persistence module which stores non-string data.
The individual submodules are described in the following sections.
源代码： Lib/dbm/gnu.py
The dbm.gnu module provides an interface to the GDBM library, similar to the dbm.ndbm module, but with additional functionality like crash tolerance.
注意
The file formats created by dbm.gnu and dbm.ndbm are incompatible and can not be used interchangeably.
Raised on dbm.gnu -specific errors, such as I/O errors. KeyError is raised for general mapping errors like specifying an incorrect key.
Open a GDBM database and return a gdbm 对象。
gdbm
filename ( 像路径对象 ) – The database file to open.
The following additional characters may be appended to control how the database is opened:
'f' : Open the database in fast mode. Writes to the database will not be synchronized.
'f'
's' : Synchronized mode. Changes to the database will be written immediately to the file.
's'
'u' : Do not lock database.
'u'
Not all flags are valid for all versions of GDBM. See the open_flags member for a list of supported flag characters.
open_flags
error – If an invalid flag argument is passed.
A string of characters the flag 参数对于 open() 支持。
gdbm objects behave similar to mappings ，但 items() and values() methods are not supported. The following methods are also provided:
items()
values()
It’s possible to loop over every key in the database using this method and the nextkey() method. The traversal is ordered by GDBM’s internal hash values, and won’t be sorted by the key values. This method returns the starting key.
nextkey()
Returns the key that follows key in the traversal. The following code prints every key in the database db , without having to create a list in memory that contains them all:
db
k = db.firstkey() while k is not None: print(k) k = db.nextkey(k)
If you have carried out a lot of deletions and would like to shrink the space used by the GDBM file, this routine will reorganize the database. gdbm objects will not shorten the length of a database file except by using this reorganization; otherwise, deleted file space will be kept and reused as new (key, value) pairs are added.
When the database has been opened in fast mode, this method forces any unwritten data to be written to the disk.
Close the GDBM database.
源代码： Lib/dbm/ndbm.py
The dbm.ndbm module provides an interface to the NDBM library. This module can be used with the “classic” NDBM interface or the GDBM compatibility interface.
警告
The NDBM library shipped as part of macOS has an undocumented limitation on the size of values, which can result in corrupted database files when storing values larger than this limit. Reading such corrupted files can result in a hard crash (segmentation fault).
Raised on dbm.ndbm -specific errors, such as I/O errors. KeyError is raised for general mapping errors like specifying an incorrect key.
Name of the NDBM implementation library used.
Open an NDBM database and return an ndbm 对象。
ndbm
filename ( 像路径对象 ) – The basename of the database file (without the .dir or .pag extensions).
.dir
.pag
ndbm objects behave similar to mappings ，但 items() and values() methods are not supported. The following methods are also provided:
3.11 版改变： 接受 像路径对象 for filename.
Close the NDBM database.
源代码： Lib/dbm/dumb.py
The dbm.dumb module is intended as a last resort fallback for the dbm module when a more robust module is not available. The dbm.dumb module is not written for speed and is not nearly as heavily used as the other database modules.
The dbm.dumb module provides a persistent dict -like interface which is written entirely in Python. Unlike other dbm backends, such as dbm.gnu , no external library is required.
The dbm.dumb module defines the following:
Raised on dbm.dumb -specific errors, such as I/O errors. KeyError is raised for general mapping errors like specifying an incorrect key.
打开 dbm.dumb database. The returned database object behaves similar to a 映射 , in addition to providing sync() and close() 方法。
sync()
close()
filename –
The basename of the database file (without extensions). A new database creates the following files:
filename.dat
filename.dir
'r' : Open existing database for reading only.
'c' (default): Open database for reading and writing, creating it if it doesn’t exist.
It is possible to crash the Python interpreter when loading a database with a sufficiently large/complex entry due to stack depth limitations in Python’s AST compiler.
3.5 版改变： open() always creates a new database when flag is 'n' .
3.8 版改变： A database opened read-only if flag is 'r' . A database is not created if it does not exist if flag is 'r' or 'w' .
In addition to the methods provided by the collections.abc.MutableMapping class, the following methods are provided:
collections.abc.MutableMapping
Synchronize the on-disk directory and data files. This method is called by the Shelve.sync() 方法。
Shelve.sync()
Close the database.
sqlite3 — SQLite 数据库的 DB-API 2.0 接口
sqlite3
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