"""PyDbLite.py
In-memory database management, with selection by list comprehension
or generator expression
Fields are untyped : they can store anything that can be pickled.
Selected records are returned as dictionaries. Each record is
identified by a unique id and has a version number incremented
at every record update, to detect concurrent access
Syntax :
from PyDbLite import Base
db = Base('dummy')
# create new base with field names
db.create('name','age','size')
# existing base
db.open()
# insert new record
db.insert(name='homer',age=23,size=1.84)
# records are dictionaries with a unique integer key __id__
# selection by list comprehension
res = [ r for r in db if 30 > r['age'] >= 18 and r['size'] < 2 ]
# or generator expression
for r in (r for r in db if r['name'] in ('homer','marge') ):
# simple selection (equality test)
res = db(age=30)
# delete a record or a list of records
db.delete(one_record)
db.delete(list_of_records)
# delete a record by its id
del db[rec_id]
# direct access by id
record = db[rec_id] # the record such that record['__id__'] == rec_id
# create an index on a field
db.create_index('age')
# access by index
records = db._age[23] # returns the list of records with age == 23
# update
db.update(record,age=24)
# add and drop fields
db.add_field('new_field')
db.drop_field('name')
# save changes on disk
db.commit()
"""
import os
import cPickle
import bisect
# compatibility with Python 2.3
try:
set([])
except NameError:
from sets import Set as set
class Index:
"""Class used for indexing a base on a field
The instance of Index is an attribute the Base instance"""
def __init__(self,db,field):
self.db = db # database object (instance of Base)
self.field = field # field name
def __iter__(self):
return iter(self.db.indices[self.field])
def keys(self):
return self.db.indices[self.field].keys()
def __getitem__(self,key):
"""Lookup by key : return the list of records where
field value is equal to this key, or an empty list"""
ids = self.db.indices[self.field].get(key,[])
return [ self.db.records[_id] for _id in ids ]
class Base:
def __init__(self,basename):
self.name = basename
def create(self,*fields,**kw):
"""Create a new base with specified field names
A keyword argument mode can be specified ; it is used if a file
with the base name already exists
- if mode = 'open' : open the existing base, ignore the fields
- if mode = 'override' : erase the existing base and create a
new one with the specified fields"""
self.mode = mode = kw.get("mode",None)
if os.path.exists(self.name):
if not os.path.isfile(self.name):
raise IOError,"%s exists and is not a file" %self.name
elif mode is None:
raise IOError,"Base %s already exists" %self.name
elif mode == "open":
return self.open()
elif mode == "override":
os.remove(self.name)
self.fields = list(fields)
self.records = {}
self.next_id = 0
self.indices = {}
self.commit()
return self
def create_index(self,*fields):
"""Create an index on the specified field names
An index on a field is a mapping between the values taken by the field
and the sorted list of the ids of the records whose field is equal to
this value
For each indexed field, an attribute of self is created, an instance
of the class Index (see above). Its name it the field name, with the
prefix _ to avoid name conflicts
"""
reset = False
for f in fields:
if not f in self.fields:
raise NameError,"%s is not a field name" %f
# initialize the indices
if self.mode == "open" and f in self.indices:
continue
reset = True
self.indices[f] = {}
for _id,record in self.records.iteritems():
# use bisect to quickly insert the id in the list
bisect.insort(self.indices[f].setdefault(record[f],[]),
_id)
# create a new attribute of self, used to find the records
# by this index
setattr(self,'_'+f,Index(self,f))
if reset:
self.commit()
def open(self):
"""Open an existing database and load its content into memory"""
_in = open(self.name) # don't specify binary mode !
self.fields = cPickle.load(_in)
self.next_id = cPickle.load(_in)
self.records = cPickle.load(_in)
self.indices = cPickle.load(_in)
for f in self.indices.keys():
setattr(self,'_'+f,Index(self,f))
_in.close()
self.mode = "open"
return self
def commit(self):
"""Write the database to a file"""
out = open(self.name,'wb')
cPickle.dump(self.fields,out)
cPickle.dump(self.next_id,out)
cPickle.dump(self.records,out)
cPickle.dump(self.indices,out)
out.close()
def insert(self,*args,**kw):
"""Insert a record in the database
Parameters can be positional or keyword arguments. If positional
they must be in the same order as in the create() method
If some of the fields are missing the value is set to None
Returns the record identifier
"""
if args:
kw = dict([(f,arg) for f,arg in zip(self.fields,args)])
# initialize all fields to None
record = dict([(f,None) for f in self.fields])
# set keys and values
for (k,v) in kw.iteritems():
record[k]=v
# add the key __id__ : record identifier
record['__id__'] = self.next_id
# add the key __version__ : version number
record['__version__'] = 0
# create an entry in the dictionary self.records, indexed by __id__
self.records[self.next_id] = record
# update index
for ix in self.indices.keys():
bisect.insort(self.indices[ix].setdefault(record[ix],[]),
self.next_id)
# increment the next __id__ to attribute
self.next_id += 1
return record['__id__']
def delete(self,removed):
"""Remove a single record, or the records in an iterable
Before starting deletion, test if all records are in the base
and don't have twice the same __id__
Return the number of deleted items
"""
if isinstance(removed,dict):
# remove a single record
removed = [removed]
else:
# convert iterable into a list (to be able to sort it)
removed = [ r for r in removed ]
if not removed:
return 0
_ids = [ r['__id__'] for r in removed ]
_ids.sort()
keys = set(self.records.keys())
# check if the records are in the base
if not set(_ids).issubset(keys):
missing = list(set(_ids).difference(keys))
raise IndexError,'Delete aborted. Records with these ids' \
' not found in the base : %s' %str(missing)
# raise exception if duplicate ids
for i in range(len(_ids)-1):
if _ids[i] == _ids[i+1]:
raise IndexError,"Delete aborted. Duplicate id : %s" %_ids[i]
deleted = len(removed)
while removed:
r = removed.pop()
_id = r['__id__']
# remove id from indices
for indx in self.indices.keys():
pos = bisect.bisect(self.indices[indx][r[indx]],_id)-1
del self.indices[indx][r[indx]][pos]
if not self.indices[indx][r[indx]]:
del self.indices[indx][r[indx]]
# remove record from self.records
del self.records[_id]
return deleted
def update(self,record,**kw):
"""Update the record with new keys and values and update indices"""
# update indices
_id = record["__id__"]
for indx in self.indices.keys():
if indx in kw.keys():
if record[indx] == kw[indx]:
continue
# remove id for the old value
old_pos = bisect.bisect(self.indices[indx][record[indx]],_id)-1
del self.indices[indx][record[indx]][old_pos]
if not self.indices[indx][record[indx]]:
del self.indices[indx][record[indx]]
# insert new value
bisect.insort(self.indices[indx].setdefault(kw[indx],[]),_id)
# update record values
record.update(kw)
# increment version number
record["__version__"] += 1
def add_field(self,field,default=None):
if field in self.fields + ["__id__","__version__"]:
raise ValueError,"Field %s already defined" %field
for r in self:
r[field] = default
self.fields.append(field)
self.commit()
def drop_field(self,field):
if field in ["__id__","__version__"]:
raise ValueError,"Can't delete field %s" %field
self.fields.remove(field)
for r in self:
del r[field]
if field in self.indices:
del self.indices[field]
self.commit()
def __call__(self,**kw):
"""Selection by field values
db(key=value) returns the list of records where r[key] = value"""
for key in kw:
if not key in self.fields:
raise ValueError,"Field %s not in the database" %key
def sel_func(r):
for key in kw:
if not r[key] == kw[key]:
return False
return True
return [ r for r in self if sel_func(r) ]
def __getitem__(self,record_id):
"""Direct access by record id"""
return self.records[record_id]
def __len__(self):
return len(self.records)
def __delitem__(self,record_id):
"""Delete by record id"""
self.delete(self[record_id])
def __iter__(self):
"""Iteration on the records"""
return self.records.itervalues()
if __name__ == '__main__':
# test on a 1000 record base
import random
import datetime
names = ['pierre','claire','simon','camille','jean',
'florence','marie-anne']
db = Base('PyDbLite_test')
db.create('name','age','size','birth',mode="override")
for i in range(1000):
db.insert(name=unicode(random.choice(names)),
age=random.randint(7,47),size=random.uniform(1.10,1.95),
birth=datetime.date(1990,10,10))
db.create_index('age')
db.commit()
print 'Record #20 :',db[20]
print '\nRecords with age=30 :'
for rec in db._age[30]:
print '%-10s | %2s | %s' %(rec['name'],rec['age'],round(rec['size'],2))
print "\nSame with __call__"
for rec in db(age=30):
print '%-10s | %2s | %s' %(rec['name'],rec['age'],round(rec['size'],2))
print db._age[30] == db(age=30)
db.insert(name=unicode(random.choice(names))) # missing fields
print '\nNumber of records with 30 <= age < 33 :',
print sum([1 for r in db if 33 > r['age'] >= 30])
print db.delete([])
d = db.delete([r for r in db if 32> r['age'] >= 30 and r['name']==u'pierre'])
print "\nDeleting %s records with name == 'pierre' and 30 <= age < 32" %d
print '\nAfter deleting records '
for rec in db._age[30]:
print '%-10s | %2s | %s' %(rec['name'],rec['age'],round(rec['size'],2))
print '\n',sum([1 for r in db]),'records in the database'
print '\nMake pierre uppercase for age > 27'
for record in ([r for r in db if r['name']=='pierre' and r['age'] >27]) :
db.update(record,name=u"Pierre")
print len([r for r in db if r['name']==u'Pierre']),'Pierre'
print len([r for r in db if r['name']==u'pierre']),'pierre'
print len([r for r in db if r['name'] in [u'pierre',u'Pierre']]),'p/Pierre'
print 'is unicode :',isinstance(db[20]['name'],unicode)
db.commit()
db.open()
print '\nSame operation after commit + open'
print len([r for r in db if r['name']==u'Pierre']),'Pierre'
print len([r for r in db if r['name']==u'pierre']),'pierre'
print len([r for r in db if r['name'] in [u'pierre',u'Pierre']]),'p/Pierre'
print 'is unicode :',isinstance(db[20]['name'],unicode)
print "\nDeleting record #20"
del db[20]
if not 20 in db:
print "record 20 removed"
print db[21]
db.drop_field('name')
print db[21]
db.add_field('adate',datetime.date.today())
print db[21]
k = db._age.keys()[0]
print "key",k
print k in db._age
db.delete(db._age[k])
print db._age[k]
print k in db._age
