[Tutor] Funtions, modules & global variables

[questions anon]

Thanks for your feedback. I might be asking the wrong questions.
I have written a number of scripts that do various things for each step:
1. get data (e.g. Dec-jan-feb, only januarys, all data)
2. summary stats(e.g. max, min, frequency)
3. plot data (e.g. plot with time, plots subplots, plot with particular
projection)

I would like to then be able to choose which script for each step and
somehow feed the data/variables written in earlier functions through to then
be able to run something like:

def main():
     getdataDJF()
     calculatemean()
     plotmean()
main()

Is this possible? Is this the correct way to do things?
My main problem is not being able to feed the output from one function into
the next.
Thanks
Sarah


On Mon, Sep 26, 2011 at 10:35 AM, Alan Gauld <alan.gauld at btinternet.com>wrote:

> On 26/09/11 00:21, questions anon wrote:
>
>> Hi All,
>> I am trying to move from writing one long script each time I need to do
>> a new thing but I do not quite understand how functions work, even after
>> reading many manuals.
>>
>
> Can you be more specific about what you dpn;t understand.
>
> For example can you understand how this function works?
>
> def square(x):
>    return x*x
>
> If not what is the area of difficulty?
>
>
>  My first attempt is to create a python file where I have defined a
>> number of functions that get data in various ways.
>> I have also made another python file where I want to plot the data in
>> different ways. but I am having trouble getting that data to run in my
>> python plotting file.
>>
>
> It may be easier to keep all the functions in a single file initially. It
> just eliminates an extra layer of complexity until we get things working.
> Once you know your functions work we can think about moving them into
> separate files.
>
>
>  I have made the variable I require 'global' but that doesn't seem to be
>> working. I keep receiving:
>> /
>> UnboundLocalError: local variable 'TSFC' referenced before assignment/
>>
>
> global in Python means global within the current file. It does not mean
> across all files.
>
>
>
>> Below is a some of the script for both folder, just note that their will
>> be many functions in each folder
>>
>
> I'm not sure I understand that bit. So I'll just ignore it for now! :-)
>
>
>
>  #selectdata.py
>> def selectalldata():
>>         for (path, dirs, files) in os.walk(MainFolder):
>>             for dir in dirs:
>>                 print dir
>>             path=path+'/'
>>             for ncfile in files:
>>                 if ncfile[-3:]=='.nc':
>>                     print "dealing with ncfiles:", path+ncfile
>>                     ncfile=os.path.join(path,**ncfile)
>>                     ncfile=Dataset(ncfile, 'r+', 'NETCDF4')
>>                     global TSFC
>>
>
> Normally globals are defined at the top of the function. Its not necessary
> but its where most readers will look for it. In this case there is no
> existing global variable TSFC so it gets created in your module space on the
> assignment.
>
>
> >                      TSFC=ncfile.variables['T_SFC']**[:,:,:]
>
> I'm not familiar with the [:,:,:] style? What does it do?
> Are you sure you want the commas?
>
>
> But since that happens inside an if block, if the block doesn't get
> executed the global variable will never get created. Any attempt to access
> the variable before the if block is executed will result in your error.
>
>
>                      LAT=ncfile.variables['**latitude'][:]
>>                     LON=ncfile.variables['**longitude'][:]
>>                     TIME=ncfile.variables['time'][**:]
>>                     fillvalue=ncfile.variables['T_**SFC']._FillValue
>>                     ncfile.close()
>>
>
> And since there are no other functions in this file nobody else can see the
> TSFC variable you created. And since the function does not return any data
> it will be hard for other modules to use this function.
>
>
>
>
>>
>> #plotdata.py
>>
>> from selectdata import selectalldata
>> selectalldata()
>>
>
> This will (msometimes) create a TSFC variable in the selectdata module, but
> you will not be able to access it here because you have not imported
> selectdata. If you had you could have done
>
> TSFC = selectdata.TSFC
>
> But since you didn't I suspect that
>
>
>  def plotncfilewithtime():
>>     for TSFC, TIME in zip((TSFC[1::3]),(TIME[1::3]))**:
>>
>
> This will fail when it tries to slice TSFC which it can't see.
> Even if it did work it would be bad practice to do this sin e from the next
> line onwards the original TSFC would be masked by the new TSFC variable you
> are creating. Better to use unique names for the two values.
>
>
>      #convert time from numbers to date and prepare it to have no
>> symbols for saving to filename
>>
> ....
> I ignored the techie stuff :-)
>
>
>      print "end of processing"
>> plotncfilewithtime()
>>
>
> Again, this is not the best way to use functions, better to get the
> function to return the output to be displayed by the other program.
>
> HTH,
>
> --
> Alan G
> Author of the Learn to Program web site
> http://www.alan-g.me.uk/
>
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