Hossein Hossein - 1 month ago 25
Python Question

No data is read when using lmdb cursor in python

I have a lmdb database and I'm trying to read its contents!

the irony is nothing gets printed on screen!

This is the code snippet that I have written for reading from lmdb :

import caffe
import lmdb
import numpy as np
from caffe.proto import caffe_pb2
import cv2
import sys

db_train = lmdb.open('mnist_train_lmdb')
db_train_txn = db_train.begin()
cursor = db_train_txn.cursor()

print db_train
print db_train_txn
print db_train_txn.cursor()

datum = caffe_pb2.Datum()

index = sys.argv[0]
size_train = 50000
size_test = 10000
data_train = np.zeros((size_train, 1, 28, 28))
label_train = np.zeros(size_train, dtype=int)

print 'Reading training data...'
i = -1
for key, value in cursor:
i = i + 1
if i % 1000 == 0:
print i
if i == size_train:
break
datum.ParseFromString(value)
label = datum.label
data = caffe.io.datum_to_array(datum)
data_train[i] = data
label_train[i] = label


This prints :

<Environment object at 0x0000000009CE3990>
<Transaction object at 0x0000000009CE1810>
<Cursor object at 0x0000000009863738>
Reading training data...
Reading test data...


It seems the for loop doesnt run at all!

What am I missing here?

I checked and it seems this is the normal way of reading from lmdb, all source examples that I have seen have similar approach.

Answer

Correct myself:

Both way of using lmdb.Cursor()

for key, value in cursor:

and

while cursor.next():

are right and I was wrong in the original answer.


You didn't use cursor properly and a slight modification should be made in your code like:

... # original stuff
print 'Reading training data...'
i = -1
while cursor.next(): #  Move to the next element, and
    i = i + 1        #  note cursor starts in an unpositioned state
    if i % 1000 == 0:
        print i
    if i == size_train:
        break
    datum.ParseFromString(cursor.value())
    label = datum.label
    data = caffe.io.datum_to_array(datum)
    data_train[i] = data
    label_train[i] = label

For more usage about lmdb python binding, you can refer here.

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