[Biopython] samtools does not return correct exit code

[Mic]

Hallo,
Samtools does not return correct the exit code:

import subprocess
import logging
import sys

def run_cmd(args):
        if subprocess.call(args,shell=True) != 0:
                print 'hello'
                logging.error("Error copying sequence file args='%s'" %
str(args))
                return 1
        print 'e', sys.stderr
        print 'o', sys.stdout
        return 0


def runSamtools( cmd ):
    '''run a samtools command'''

    try:
        retcode = subprocess.call(cmd, shell=True)
        print retcode
        if retcode < 0:
            print >>sys.stderr, "Child was terminated by signal", -retcode
    except OSError, e:
        print >>sys.stderr, "Execution failed:", e

print run_cmd("samtools faidx ex1.fa")
print runSamtools("samtools faidx ex1.fa")

print 'Hello still alive'


and as output I got:

$ python p3.py
open: No such file or directory
[_razf_open] fail to open ex1.fa
[fai_build] fail to open the FASTA file ex1.fa
e <open file '<stderr>', mode 'w' at 0x7ffa4658d270>
o <open file '<stdout>', mode 'w' at 0x7ffa4658d1e0>
0
open: No such file or directory
[_razf_open] fail to open ex1.fa
[fai_build] fail to open the FASTA file ex1.fa
0
None
Hello still alive

How can I get sure that all samtools commands were executed successfully?

Thank you in advance.