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+[Contents](../Contents.md) \| [Previous (6.3 Producer/Consumer)](03_Producers_consumers.md) \| [Next (7 Advanced Topics)](../07_Advanced_Topics/00_Overview.md)
+
+# 6.4 More Generators
+
+This section introduces a few additional generator related topics
+including generator expressions and the itertools module.
+
+### Generator Expressions
+
+A generator version of a list comprehension.
+
+```python
+>>> a = [1,2,3,4]
+>>> b = (2*x for x in a)
+>>> b
+<generator object at 0x58760>
+>>> for i in b:
+...   print(i, end=' ')
+...
+2 4 6 8
+>>>
+```
+
+Differences with List Comprehensions.
+
+* Does not construct a list.
+* Only useful purpose is iteration.
+* Once consumed, can't be reused.
+
+General syntax.
+
+```python
+(<expression> for i in s if <conditional>)
+```
+
+It can also serve as a function argument.
+
+```python
+sum(x*x for x in a)
+```
+
+It can be applied to any iterable.
+
+```python
+>>> a = [1,2,3,4]
+>>> b = (x*x for x in a)
+>>> c = (-x for x in b)
+>>> for i in c:
+...   print(i, end=' ')
+...
+-1 -4 -9 -16
+>>>
+```
+
+The main use of generator expressions is in code that performs some
+calculation on a sequence, but only uses the result once.  For
+example, strip all comments from a file.
+
+```python
+f = open('somefile.txt')
+lines = (line for line in f if not line.startswith('#'))
+for line in lines:
+    ...
+f.close()
+```
+
+With generators, the code runs faster and uses little memory. It's
+like a filter applied to a stream.
+
+### Why Generators
+
+* Many problems are much more clearly expressed in terms of iteration.
+  * Looping over a collection of items and performing some kind of operation (searching, replacing, modifying, etc.).
+  * Processing pipelines can be applied to a wide range of data processing problems.
+* Better memory efficiency.
+  * Only produce values when needed.
+  * Contrast to constructing giant lists.
+  * Can operate on streaming data
+* Generators encourage code reuse
+  * Separates the *iteration* from code that uses the iteration
+  * You can build a toolbox of interesting iteration functions and *mix-n-match*.
+
+### `itertools` module
+
+The `itertools` is a library module with various functions designed to help with iterators/generators.
+
+```python
+itertools.chain(s1,s2)
+itertools.count(n)
+itertools.cycle(s)
+itertools.dropwhile(predicate, s)
+itertools.groupby(s)
+itertools.ifilter(predicate, s)
+itertools.imap(function, s1, ... sN)
+itertools.repeat(s, n)
+itertools.tee(s, ncopies)
+itertools.izip(s1, ... , sN)
+```
+
+All functions process data iteratively.
+They implement various kinds of iteration patterns.
+
+More information at [Generator Tricks for Systems Programmers](http://www.dabeaz.com/generators/) tutorial from PyCon '08.
+
+## Exercises
+
+In the previous exercises, you wrote some code that followed lines being written to a log file and parsed them into a sequence of rows.
+This exercise continues to build upon that.  Make sure the `Data/stocksim.py` is still running.
+
+### Exercise 6.13: Generator Expressions
+
+Generator expressions are a generator version of a list comprehension.
+For example:
+
+```python
+>>> nums = [1, 2, 3, 4, 5]
+>>> squares = (x*x for x in nums)
+>>> squares
+<generator object <genexpr> at 0x109207e60>
+>>> for n in squares:
+...     print(n)
+...
+1
+4
+9
+16
+25
+```
+
+Unlike a list a comprehension, a generator expression can only be used once.
+Thus, if you try another for-loop, you get nothing:
+
+```python
+>>> for n in squares:
+...     print(n)
+...
+>>>
+```
+
+### Exercise 6.14: Generator Expressions in Function Arguments
+
+Generator expressions are sometimes placed into function arguments.
+It looks a little weird at first, but try this experiment:
+
+```python
+>>> nums = [1,2,3,4,5]
+>>> sum([x*x for x in nums])    # A list comprehension
+55
+>>> sum(x*x for x in nums)      # A generator expression
+55
+>>>
+```
+In the above example, the second version using generators would
+use significantly less memory if a large list was being manipulated.
+
+In your `portfolio.py` file, you performed a few calculations
+involving list comprehensions.  Try replacing these with
+generator expressions.
+
+### Exercise 6.15: Code simplification
+
+Generators expressions are often a useful replacement for
+small generator functions.  For example, instead of writing a
+function like this:
+
+```python
+def filter_symbols(rows, names):
+    for row in rows:
+        if row['name'] in names:
+            yield row
+```
+
+You could write something like this:
+
+```python
+rows = (row for row in rows if row['name'] in names)
+```
+
+Modify the `ticker.py` program to use generator expressions
+as appropriate.
+
+
+[Contents](../Contents.md) \| [Previous (6.3 Producer/Consumer)](03_Producers_consumers.md) \| [Next (7 Advanced Topics)](../07_Advanced_Topics/00_Overview.md)