Python Hour - 29 September 2014

• Parker brought our attention to a programming book called Introduction to Computation and Programming Using Python, by John Guttag. The book is based on a course offered on MIT's OpenCourseWare.

• We spent the some time talking about what happens when you try to create a netcdf file that already exists. By default, when you open a netcdf dataset with mode='w' (i.e., in writing mode) it should over-write any existing file. You can change this behavior with the "clobber" keyword option, which can raise an exception if the file already exists:

  import netCDF4 as nc
  
  #create new netcdf file and over-write any pre-existing file with same name
  h = nc.Dataset('filename.nc', mode='w')
  
  #create new netcdf file but throw an error if the file already exists
  h = nc.Dataset('filename.nc', mode='w', clobber=False)
  

• We also learned that it is possible extract a subset of a netcdf file stored on a remote THREDDS data server WITHOUT downloading the entire file. You simply supply the url path to the netcdf file and extract data as you would normally.

• Jim spoke about some of the advantages and disadvantages of using a Python set. A Python set is an unordered collection of unique objects. This is in contrast to a Python list, which is an ordered sequence of objects that may contain duplicates. One advantage of using a set over a list is that a set is substantially faster at determining if an object is in it. For example, performing if x in set is generally faster than doing if x in list. One disadvantage of using sets is that they have no intrinsic order:

  s = set("aoihf")    
  for n in s:
      print n 
  #output: 
  a i f o h
  

This will cause problems if you are iterating over a set and the order in which the set's items are accessed is important.

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