Here is a minimal script called mpi_test.py:

from yt.mods import *
ds = load("/scratch/cerberus/d5/ctss/cosmoSim_metal_run_mn2_lmax22_hdf5_chk_0108")
v, c = ds.find_max("density")
print 'Max density is', v, 'located at', c

Here is the output when run in in series:

(yt-x86_64)[asmith@r900-1 scripts_yt]$ python2.7 mpi_test.py 
yt : [WARNING  ] 2014-04-22 11:42:40,726 integer runtime parameter checkpointfilenumber overwrites a simulation scalar of the same name
yt : [WARNING  ] 2014-04-22 11:42:40,727 integer runtime parameter forcedplotfilenumber overwrites a simulation scalar of the same name
yt : [WARNING  ] 2014-04-22 11:42:40,727 integer runtime parameter nbegin overwrites a simulation scalar of the same name
yt : [WARNING  ] 2014-04-22 11:42:40,727 integer runtime parameter particlefilenumber overwrites a simulation scalar of the same name
yt : [WARNING  ] 2014-04-22 11:42:40,727 integer runtime parameter plotfilenumber overwrites a simulation scalar of the same name
yt : [INFO     ] 2014-04-22 11:42:40,758 Parameters: current_time              = 9.82976564106e+15
yt : [INFO     ] 2014-04-22 11:42:40,758 Parameters: domain_dimensions         = [8 8 8]
yt : [INFO     ] 2014-04-22 11:42:40,759 Parameters: domain_left_edge          = [ 0.  0.  0.]
yt : [INFO     ] 2014-04-22 11:42:40,759 Parameters: domain_right_edge         = [  3.08567758e+24   3.08567758e+24   3.08567758e+24]
yt : [INFO     ] 2014-04-22 11:42:40,760 Parameters: cosmological_simulation   = 1
yt : [INFO     ] 2014-04-22 11:42:40,760 Parameters: current_redshift          = 13.8092390704
yt : [INFO     ] 2014-04-22 11:42:40,760 Parameters: omega_lambda              = 0.728
yt : [INFO     ] 2014-04-22 11:42:40,760 Parameters: omega_matter              = 0.272
yt : [INFO     ] 2014-04-22 11:42:40,760 Parameters: hubble_constant           = 0.704000425774
yt : [INFO     ] 2014-04-22 11:42:57,208 Loading field plugins.
yt : [INFO     ] 2014-04-22 11:42:57,209 Loaded angular_momentum (8 new fields)
yt : [INFO     ] 2014-04-22 11:42:57,209 Loaded astro (14 new fields)
yt : [INFO     ] 2014-04-22 11:42:57,209 Loaded cosmology (20 new fields)
yt : [INFO     ] 2014-04-22 11:42:57,210 Loaded fluid (55 new fields)
yt : [INFO     ] 2014-04-22 11:42:57,211 Loaded fluid_vector (87 new fields)
yt : [INFO     ] 2014-04-22 11:42:57,211 Loaded geometric (102 new fields)
yt : [INFO     ] 2014-04-22 11:42:57,211 Loaded local (102 new fields)
yt : [INFO     ] 2014-04-22 11:42:57,212 Loaded magnetic_field (108 new fields)
yt : [INFO     ] 2014-04-22 11:44:02,644 Max Value is 1.78281e-17 at 1544218471251428570562560.0000000000000000 1433864018495179847630848.0000000000000000 1638997414248496788144128.0000000000000000
Max density is 1.78280946468e-17 g/cm**3 located at [  1.54421847e+24   1.43386402e+24   1.63899741e+24]

Here is the output when run in parallel:

(yt-x86_64)[asmith@r900-1 scripts_yt]$ mpirun -np 2 python2.7 mpi_test.py --parallel
Fatal error in MPI_Init_thread: Other MPI error, error stack:
MPIR_Init_thread(294): Initialization failed
MPID_Init(74)........: 
MPICH_CH3_PreLoad(63): 
MPIU_DLL_Open(42)....:  Unable to load the dynamically loadable library libmpich2-ch3-sock.so for reason /usr/lib64/mpich2/libmpich2-ch3-sock.so: undefined symbol: MPIDI_Process[unset]: aborting job:
Fatal error in MPI_Init_thread: Other MPI error, error stack:
MPIR_Init_thread(294): Initialization failed
MPID_Init(74)........: 
MPICH_CH3_PreLoad(63): 
MPIU_DLL_Open(42)....:  Unable to load the dynamically loadable library libmpich2-ch3-sock.so for reason /usr/lib64/mpich2/libmpich2-ch3-sock.so: undefined symbol: MPIDI_Process
Fatal error in MPI_Init_thread: Other MPI error, error stack:
MPIR_Init_thread(294): Initialization failed
MPID_Init(74)........: 
MPICH_CH3_PreLoad(63): 
MPIU_DLL_Open(42)....:  Unable to load the dynamically loadable library libmpich2-ch3-sock.so for reason /usr/lib64/mpich2/libmpich2-ch3-sock.so: undefined symbol: MPIDI_Process[unset]: aborting job:
Fatal error in MPI_Init_thread: Other MPI error, error stack:
MPIR_Init_thread(294): Initialization failed
MPID_Init(74)........: 
MPICH_CH3_PreLoad(63): 
MPIU_DLL_Open(42)....:  Unable to load the dynamically loadable library libmpich2-ch3-sock.so for reason /usr/lib64/mpich2/libmpich2-ch3-sock.so: undefined symbol: MPIDI_Process