Encounter an error while it is installing one dependence of the py-matplotlib

[心如烛光]

Hey,guys!
     I encounter an error while it is installing py27-pyobjc,which is one of the py-matplotlib's dependence.What it returns is:"command execution failed".I wanna know that what may causes the so call "command execution failed".And how could I fix it?Thank's a lot!!
Here is the return:
--->  Staging py27-pyobjc into destroot
Error: org.macports.destroot for port py27-pyobjc returned: command execution failed
Error: Failed to install py27-pyobjc
Please see the log file for port py27-pyobjc for details:
    /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_python_py-pyobjc/py27-pyobjc/main.log
Error: The following dependencies were not installed: py27-matplotlib py27-pyobjc-cocoa py27-pyobjc py27-six py27-tornado py27-backports-ssl_match_hostname py27-backports
To report a bug, follow the instructions in the guide:
    http://guide.macports.org/#project.tickets
Error: Processing of port py-matplotlib failed



And here is part of the log file for port py27-pyobjc:
  _opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_python_py-pyobjc/py27-pyobjc/work/destroot   :info:destroot Exit code: 1  :error:destroot org.macports.destroot for port py27-pyobjc returned: command execution failed  :debug:destroot Error code: CHILDSTATUS 6925 1  :debug:destroot Backtrace: command execution failed      while executing  "system -nice 0 $fullcmdstring"      ("eval" body line 1)      invoked from within  "eval system $notty $nice \$fullcmdstring"      invoked from within  "command_exec destroot"      (procedure "portdestroot::destroot_main" line 2)      invoked from within  "portdestroot::destroot_main org.macports.destroot"      ("eval" body line 1)      invoked from within  "eval $procedure $targetname"  :info:destroot Warning: targets not executed for py27-pyobjc: org.macports.activate org.macports.destroot org.macports.install  :notice:destroot Please see the log file for port py27-pyobjc for details:      /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_python_py-pyobjc/py27-pyobjc/main.log
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