Python – 数据库管理器 (dbm) 包

在本文中，我们将了解 dbm，它是Python内置库中的一个包。 Python中的 dbm 包提供了一个类似字典的简单接口，格式为 DBM（数据库管理器），通常用于 Unix 操作系统。 dbm 以简单的键值对形式存储数据，就像字典一样，这使得从数据库中插入、编辑和检索数据变得更加容易。它通过使用单个主键（“键”）将数据存储在固定大小的块中。

dbm 包中包含三种类型的子模块：

1. dbm.gnu： GNU 对 dbm 的重新解释
2. dbm.ndbm：基于ndbm的接口
3. dbm.dumb：可移植的 DBM 实现

以下是 dbm 包中可用的主要功能：

dbm.open()

该函数用于打开一个 dbm 数据库，如果不存在则创建一个新数据库。

Syntax: dbm.open(file, flag=’r’, mode=0o666)

Parameters: This function take following parameters: 

• file: name of the file.
• flag: mode of permissions. which can be ‘r’, ‘w’, ‘c’, or ‘n’.
  • ‘r’: open the existing database with permission to read only.
  • ‘w’: open the existing database with permission to read and write.
  • ‘c’: open the database for read and write, also create a new one if it doesn’t exists.
  • ‘n’: Always create a new database with permission to both read and write.
• mode: The Unix mode of the argument which is a octal form default set to 0o666, used only when new database is to be created.

Return: The corresponding object address of the database file. 

dbm.whichdb()

此函数尝试猜测几个可用的简单数据库模块（dbm.gnu、dbm.ndbm 或 dbm.dumb）中的哪一个应该用于打开给定文件。

Syntax: dbm.whichdb(filename) 

Parameter: filename- Name of the file.

Returns: The function returns one of the following values :

• None: If the database doesn’t exists or it can’t be opened.
• (‘ ‘): An empty string, if the file exists but the file format can’t be guessed else
• The required module name: If the type is successfully detected then one of the string names is returned, ‘dbm.gnu’, ‘dbm.ndbm’ or ‘dbm.dumb’.

以下是 dbm 对象的内置方法：

• items(): This method returns the items contained in the database of caller object in form of key value pairs. (db is the caller object database).
• clear(): clear all the values present in the database.
• get(key): returns the value corresponding to key given in argument.
• keys(), iterkeys(): returns an iterable list containing keys of the dictionary.
• pop(key): Deletes / pops the key, value pair corresponding to key given in argument.
• setdefault(): set a default primary key given in the argument.
• sync(): Helps to synchronize data files and on disk directory.
• update(): updates the existing key value. Just like dictionary object.
• values(): iterate through all the values present in database.
• close(): Doesn’t take any argument nor returns anything. Just closes the caller object database. (db in this case)

以下是上述所有讨论方法/功能的实现：

代码：

Python3

# importing the dbm package
import dbm
 
# using the open function
# to create a new database named
# 'mydb' in 'n' mode, object
# returned in db variable.
db = dbm.open('mydb','n')
 
# inserting the new key and
# values in the database.
db['name'] = 'GeeksforGeeks'
db['phone'] = '8888'
db['Short name'] = 'GfG'
db['Date'] = '01/01/2000'
 
# getting and printing
# the value through get method.
print(db.get('name'))
print()
 
# printing the values of
# database through values()
# method (iterator).
for value in db.values():
    print(value)
print()
 
# printing the values through
# key iterator.
for key in db.keys():
    print(db.get(key))
print()
 
# poping out the key, value
# pair corresponding to
# 'phone' key.
db.pop('phone')
 
# printing the key, value
# pairs present in database.
for key, value in db.items():
    print(key, value)
 
# clearing all the key values
# in database.
db.clear()
 
# Below loop will print nothing
# as database is cleared above.
for key, value in db.items():
    print(key, value)
 
# closing the database.
db.close()
 
# This code is contributed by Amit Mangal.

输出 ：

b'GeeksforGeeks'

b'GeeksforGeeks'
b'8888'
b'GfG'
b'01/01/2000'

b'GeeksforGeeks'
b'8888'
b'GfG'
b'01/01/2000'

b'name' b'GeeksforGeeks'
b'Short name' b'GfG'
b'Date' b'01/01/2000'