C.carouge at 01:04, 12 April 2022

2022-04-12T01:04:53Z

C.carouge: Created page with "

= Error in Python = The person was trying to reproduce an example Python program. It output the error:

AttributeError: 'function' object has no attribute ‘comput..."

2022-04-12T00:37:39Z

Created page with "<div> = Error in Python = The person was trying to reproduce an example Python program. It output the error: <pre>AttributeError: 'function' object has no attribute ‘comput..."

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= Error in Python =

The person was trying to reproduce an example Python program. It output the error:
<pre>AttributeError: 'function' object has no attribute ‘compute’ </pre>

This happened because the person had omitted the parentheses on a previous call to another function. For example, the code below will return the same error:
<pre>a=xr.DataArray([1,2])
b=a.mean
b.mean()

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'function' object has no attribute 'mean'</pre>

On the second line, <code>b=a.mean</code> will return a reference to the meanfunction associated with the DataArray <code>a</code>. In other words, <code>b</code> is the mean function, not its result. That is why the error message starts with <code>‘function’ object</code>. 

To get the results from the function, you need to call the function which is done with the <code>()</code> and any argument you need to give.

= Avoiding repetition in python through code organisation =

It is quite common when starting to write a code to just copy and paste a group of lines if we want, for example, to execute the same calculation on a different variable. If you find that you are doing that a lot, it might be time to go back and re-organise your code. A better organised code is not only more readable but is easier to fix should you find a bug. A well-organised code will limit the number of copied lines so you only have to fix your code in one point.

One simple step is to try to create a function for every block of code you are repeating, using only the variables and parameters that might change between calls as arguments.    
<pre>def myfunction( row, array ):

param = … # this doesn’t change no need to use it as an argument for the function
… Your calculation …

return newarray </pre>

In the case we looked at, one function was applied to each row of a pandas dataframe. To do so we can use the pandas dataframe <code>itterrows()</code> method and append each result to a list.   
<pre>results = []
for index, row in df.iterrows():
results.append( myfunction(row, array) ) </pre>

Basically <code><kbd>iterrows()</kbd></code> loops across each row of the dataframe and returns the index and the row itself. 

The results list can then be concatenated into one dataArray
<pre>final = xarray.concat(results, dim='mydim') </pre>

Another example we looked at was a function to create a subplot, which was then called on different variables, each representing a different region. In such a case, to handle different titles and parameters it can be handy to use a dictionary.  
<pre>regions = {'east': [mslp_east, 1, 'Eastern Australian NRM region'],
‘north’: [mslp_north, 2, 'Northern Australian NRM region'], … } </pre>

Then you can use the key/values pair to call the same function on each region:  
<pre>for k in regions.keys():
fig = subplot(regions[k][0], regions[k][1], regions[k][2]) </pre>

The dictionary method <code>keys()</code> here works in a similar way to <code>iterrows()</code> but returns only the key. Use <code>items() </code>if you want the key,value pairs and <code>values()</code> if you want only the values.

= Count the number of elements that follow a given condition =

A question was to count how many events with positive values there are for each grid point given an array like <code>anom(event, lat, lon)</code>. xarray has a function <code>count()</code>. This will count the number of valid values along the given dimensions. So we need to filter out the negative values using <code>where()</code> and then <code>count()</code> the number of values left along the "event" dimension.
<pre>positive_count = anom.where(anom > 0).count(dim=’event’)</pre>
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CodeBreak 30/03/2022 - Revision history

C.carouge at 01:04, 12 April 2022

C.carouge: Created page with "
= Error in Python = The person was trying to reproduce an example Python program. It output the error:
AttributeError: 'function' object has no attribute ‘comput..."

@@ Line 1: / Line 1: @@
 = Error in Python =
@@ Line 54: / Line 53: @@
 A question was to count how many events with positive values there are for each grid point given an array like <code>anom(event, lat, lon)</code>. xarray has a function <code>count()</code>. This will count the number of valid values along the given dimensions. So we need to filter out the negative values using <code>where()</code> and then <code>count()</code> the number of values left along the "event" dimension.
 <pre>positive_count = anom.where(anom > 0).count(dim=’event’)</pre>