StreamsStreamsUsing Python

ByBy Tim WalshTim Walsh

What is a What is a streamstream??Called a “generator” in PythonSimilar to a listFirst made available in Python

2.2, all newer versions make use of this feature

Application of StreamsApplication of StreamsA common application of streams

in other languages is for input/output.

The sys module is where the standard input, output, and error streams can be found in Python:

sys.stdinsys.stdoutsys.stderr

Application of StreamsApplication of StreamsUsing the input and output streams

one can read/write text from/to a file.

At the command line:

~\testFile.py < input.txt~\testFile.py > output.txt

On some windows machines you must specify “python before the file name as in:

~\python testFile.py > input.txt

Application of StreamsApplication of StreamsSimilarly, python programs can be

chained together so that the output from one program is the input to another

~\write.py | read.py

Now the programs are connected, read will take the output from write as its input.

Applications of StreamsApplications of Streamsknown as ‘generators’known as ‘generators’

A stream is similar to a list◦Allows a user to deal with very large

problems without waiting for the entire range of the list to be constructed first

A function that contains a yield statement is called a generator function

A generator function is able to construct the list as needed (specified by the user)

The The Yield Yield StatementStatementYield is a keyword in Python

Yield replaced the required declaration that made generators possible

from __future__ import generators

(No longer needed, just use yield)

Yield can be thought of as a ‘future statement’ and makes generators possible in Python

Applications of StreamsApplications of Streamsknown as ‘generators’known as ‘generators’

A generator is a function which can:◦stop whatever it is doing at an

arbitrary point (the yield statement)◦Return a value back to the function◦Resume from the point it had `yielded

at' later on in the function This is accomplished because ‘yield’

automatically will call the .next() function .next() will get the next element from the

generator-iterator

ExampleExampleProblem:

◦Find the first few prime numbers between a large interval of numbers

Conventional methods require a long list of primes to be generated

Solution◦Using a stream or generator in Python

the problem requires very few calculations, focusing only on the pieces of the computation desired