Using a generator to process a big file
Functions, Modules and Packages example from a Well House Consultants training course
More on Functions, Modules and Packages [link]
Source code: dfg Module: Y105
Last year, you had a good crop of apples on your tree .... what
did you do with them? Make apple pies for all the neighbours! How?
You collect all the apples and bring them into the kitchen.
You prepare the apples ready for the pies.
You make the pastry and apple pies.
You cook the pies
You distribute them.
This year, you had a huge crop of apples, and you wanted to make
apple pies ... not for all the neighbours, but for the whole
town. How did you do that? The same way that you did last
year? Alas, no. There were far too many appled to bring into
the kitchen, all your bowls would have been filled with apples
long before you had prepared them all, and your oven has only
got limited capacity. Far better to collect a few, prepare a
few, make a few pies, and distribute them ... and keep repeating
that until you have fed the whole town.
Which is what a generator function does in Python. When
you call a generator function (as the list provider in a for
statement) in Python, it will yield when it has the first result
available, which you can then process. Come back to the for
statement, and it will continue to run the generator code
('pick the next batch of apples') and return the next result,
time after time, until you're done.
Here's an example that's doing just this - opening a file in
a generator function, and returning at each call the next line
containing the word "Python". The loop in the main code is
then extracting the person's name from the line, printing it
out, and going back to the generator for the next "Python" line.
fh = open (file,"r")
# for line in fh.xreadlines():
for line in fh.readlines(): # Use previous line Python 2
if (line.find("Python") > 0):
for staff in tap("requests.xyz"):
els = staff.split(" ")
""" Sample output:
<generator object tap at 0x100636630>
WomanWithCat:y105 grahamellis$ python2 dfg
<generator object tap at 0x10b8d3d70>
Learn about this subject
This module and example are covered on the following public courses:
* Learning to program in Python
* Python Programming
Also available on on site courses for larger groups
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