Popen with python read and write blocks in python2.7

Question

I ran into a similar problem that another @AMisra user posted some time ago but did not receive a definitive answer. The problem is that I use third-party code and cannot change it too much. I have a function that is called many times in code to create a subprocess, write data to stdin and then read it. He just hangs on this line

line = self.classifier.stderr.readline()

Note that I cannot use communicate: since I func2(classifier, vectors)need to call it several times, it throws an exception.

import subprocess
import paths
import os.path

class CRFClassifier:
    def __init__(self, name, model_type, model_path, model_file, verbose):
        self.verbose = verbose
        self.name = name
        self.type = model_type
        self.model_fname = model_file
        self.model_path = model_path

        if not os.path.exists(os.path.join(self.model_path, self.model_fname)):
            print 'The model path %s for CRF classifier %s does not exist.' % (os.path.join(self.model_path, self.model_fname), name)
            raise OSError('Could not create classifier subprocess')


        self.classifier_cmd = '%s/crfsuite-stdin tag -pi -m %s -' % (paths.CRFSUITE_PATH, 
                             os.path.join(self.model_path, self.model_fname))
#        print self.classifier_cmd
        self.classifier = subprocess.Popen(self.classifier_cmd, shell = True, stdin = subprocess.PIPE, stderr = subprocess.PIPE)

        if self.classifier.poll():
            raise OSError('Could not create classifier subprocess, with error info:\n%s' % self.classifier.stderr.readline())

        #self.cnt = 0


    def classify(self, vectors):
#        print '\n'.join(vectors) + "\n\n"

        self.classifier.stdin.write('\n'.join(vectors) + "\n\n")

        lines = []
        line = self.classifier.stderr.readline()
        while (line.strip() != ''):
#            print line
            lines.append(line)
            line = self.classifier.stderr.readline()


        if self.classifier.poll():
            raise OSError('crf_classifier subprocess died')

        predictions = []
        for line in lines[1 : ]:
            line = line.strip()
#            print line
            if line != '':
                fields = line.split(':')
#                print fields
                label = fields[0]
                prob = float(fields[1])
                predictions.append((label, prob))

        seq_prob = float(lines[0].split('\t')[1])

        return seq_prob, predictions


    def poll(self):
        """
        Checks that the classifier processes are still alive
        """
        if self.classifier is None:
            return True
        else:
            return self.classifier.poll() != None

- , , . , . .

def func2():
    classifier=create a classifier object for an input file, this executes the external command
    for sentence in sentences:
        vectors=process(sentence)# some external function
        classifier.classify(features)  

Answer 1

self.classifier classify Popen(). stdout stderr.