btree – Simple BTree Database¶
This module is based on the BerkelyDB library 1.xx edition.
Examples:
import btree
# First, we need to open a stream which holds a database
# This is usually a file, but can be in-memory database
# using uio.BytesIO, a raw flash partition, etc. used of uio.BytesIO，
# Oftentimes, you want to create a database file if it doesn't
# exist and open if it exists. Idiom below takes care of this.
# In general, if there is no database file, you need to create one; if there is already one, just open it. The following idioms take this into account.
# DO NOT open database with "a+b" access mode.
try:
f = open("mydb", "r+b")
except OSError:
f = open("mydb", "w+b")
# Now open a database
db = btree.open(f)
# The keys you add will be sorted internally in the database
db[b"3"] = b"three"
db[b"1"] = b"one"
db[b"2"] = b"two"
# Assume that any changes are cached in memory unless
# explicitly flushed (or database closed). Flush database
# at the end of each "transaction".
# Any changes are assumed to be cached in memory unless explicitly refreshed (or the database is shut down). Refresh the database at the end of each process.
db.flush()
# Prints b'two'
print(db[b"2"])
# Iterate over sorted keys in the database, starting from b"2"
# until the end of the database, returning only values.
# Iterate over the sorted keys in the database, starting from B "2" to the end of the database, and only return values.
# Mind that arguments passed to values() method are *key* values.
# Prints:
# b'two'
# b'three'
for word in db.values(b"2"):
print(word)
del db[b"2"]
# No longer true, prints False
print(b"2" in db)
# Prints:
# b"1"
# b"3"
for key in db:
print(key)
db.close()
# Don't forget to close the underlying stream!
f.close()
Function¶
-
open(stream, *, flags=0, cachesize=0, pagesize=0, minkeypage=0)¶ Open a database from a random access ``stream``(similar to an open file). All other parameters are optional, are only keywords, and allow adjustment of advanced parameters of database operation (most users do not need this):
- flags - currently unused
- cachesize - Recommended maximum memory cache size in bytes. For a board with sufficient memory, using a larger value may improve performance. This value is only the recommended value. If the value is set too low, the module may occupy more memory.
- pagesize - The page size used for the node in the BTree. The acceptable range is 512-65536. If 0, the size of the underlying I/O block is used (best coordination between memory usage and performance).
- minkeypage - The minimum number of keys stored per page. The default is 0 equals to 2.
Returns a BTree object that implements a dictionary protocol (method set) and the following additional methods.
Method¶
-
btree.close()¶ Close the database. Closing the database at the end of processing is mandatory because some unwritten data may remain in the cache. Note: This does not close the underlying flow that is opened with the database, which should be closed separately (this is also mandatory to ensure that data flushed from the buffer goes into the underlying storage).
-
btree.flush()¶ Refresh any data in the cache to the underlying stream.
-
btree.__getitem__(key)¶ -
btree.get(key, default=None)¶ -
btree.__setitem__(key, val)¶ -
btree.__detitem__(key)¶ -
btree.__contains__(key)¶ Standard dictionary method.
-
btree.__iter__()¶ B-tree objects can be iterated directly (similar to dictionaries) to access all keys orderly.
-
btree.keys([start_key[, end_key[, flags]]])¶ -
btree.values([start_key[, end_key[, flags]]])¶ -
btree.items([start_key[, end_key[, flags]]])¶ These methods are similar to the standard dictionary methods, but you can also use optional parameters to iterate over the key subscope instead of the entire database. Note：This is the third method这, start_key and end_key parameters all represent key values. For example, the
values()method iterates over the values corresponding to a given key range. No start_key value means “from the first key”, No “end_key value or none means “until the end of the database”. By default, the range includes start_key ，excluding end_key , you can pass btree.INCL to set end_key to include in the iteration. Can pass btree.DESC tag to iterate in the direction of the down key. Tag value can be the same as ORed.