Programming with Python - Week 3

Programming with
Python
Week 3
Dept. of Electrical Engineering and Computer Science
Academic Year 2010-2011

Week 2 - Highlights

• Dictionaries:
{key:value,...}
• Lists:
[12,4,56,...]
• Tuples:
(ﬁrstname,lastname)=(“Programming”,”Python”)

4.1 The power of
Introspection
• Introspection is code looking at other modules and
functions in memory as objects, getting information
about them, and manipulating them.
• Along the way:
i. you'll deﬁne functions with no name,
ii. you’ll call functions with arguments out of order,
iii. you’ll reference functions whose names you don't
even know ahead of time.

Introspection at work

def info(object, spacing=10, collapse=1):
"""Print methods and doc strings.

Takes module, class, list, dictionary, or string."""
methodList = [method for method in dir(object) if callable(getattr(object, method))]
processFunc = collapse and (lambda s: " ".join(s.split())) or (lambda s: s)
print "n".join(["%s %s" %
(method.ljust(spacing),
processFunc(str(getattr(object, method).__doc__)))
for method in methodList])

(setup)

• >>> import apihelper
• >>> apihelper.info.__doc__
'Print methods and doc strings.n n Takes module,
class, list, dictionary, or string.'

4.2 Using Optional and
Named Arguments

• Python allows function arguments to have default values.
• If the function is called without the argument, the
argument gets its default value.
• Arguments can be speciﬁed
- in any order
- by using named arguments.

spacing and collapse are
optional arguments.

def info(object, spacing=10, collapse=1):

This looks totally whacked
until you realize that
arguments are simply a
dictionary.
• info(spacing=15, object=odbchelper)

4.3 Some built-in functions

• type
• str
• dir
• callable

type function

• The type function returns the datatype of any arbitrary
object.
• The possible types are listed in the types module. This is
useful for helper functions that can handle several types
of data.

Examples

• >>> type(apihelper)
<type 'module'>
• >>> type(info)
<type 'function'>
• >>> type("hello")
<type 'str'>
• >>> type(1)
<type 'int'>

types module

• >>> import types
• >>> types.ModuleType == type(apihelper)
True

str function
• The str coerces data into a string. Every datatype can be
coerced into a string.
• >>> str(1)
'1'
• >>> str(apihelper)
"<module 'apihelper' from '.../Week 3/Code/
apihelper.py'>"
• >>> str([“mortal”, “kombat”, “go and ﬁght”])
ask in class..

dir function

• dir returns a list of the attributes and methods
of any object: modules, functions, strings, lists,
dictionaries...

pretty much anything.

Examples

• >>> dir([])
• >>> dir({})
• >>> dir(apihelper)

callable function

• The callable function takes any object and returns
True if the object can be called,
or False otherwise.
example class
• Callable objects include
functions,
class methods,
even classes themselves.

callable is beautiful:

• By using the callable function on each of an object's
attributes, you can determine which attributes you care
about (methods, functions, classes) and which you want
to ignore (constants and so on)
without knowing anything about the object
ahead of time.

HINT: Introspection

Examples

• >>> import string
• >>> string.punctuation
'!"#$%&'()*+,-./:;<=>?@[]^_`{|}~'
• >>> string.join
<function join at 0x6efaf0>
• >>> callable(string.punctuation)
False
• >>> ask in class for string.join test..

4.4 Getting object
references with getattr

• You already know that Python functions are
objects.
• What you don't know is that you can get a reference to
a function without knowing its name until run-time, by
using the getattr function.

Examples

• >>> li = [“Larry”, “Brackety”]
• >>> li.pop
• >>> getattr(li, “pop”)
• >>> getattr(li, “append”)(“Junior”)
• >>> li

Ask in class..

• 1. First import the ask_in_class module
and then;
2. call the mockfunction using getattr function.

4.4.2 Introspection,
Dispatching

• what is the typical use of getattr-like functionality?

Model, View, Controller (MVC)

So, what the hell is MVC?

Dispatcher

Dispatcher
sitting here

4.4.2 Introspection,
Dispatching

• A common usage pattern of getattr is as a dispatcher.
• For example, if you had a program that could output
data in a variety of different formats, you could deﬁne
separate functions for each output format and use a
single dispatch function to call the right one.

Example

• For example, let's imagine a program that prints site
statistics in HTML, XML, and plain text formats.
• The choice of output format could be specified on the
command line, or stored in a configuration file.
• A statsout module defines three functions,
output_html, output_xml, and output_text. Then the
main program defines a single output function, like this:

Example

import statsout

def output(data, format="text"):
output_function = getattr(statsout, "output_%s" % format)
return output_function(data)

Default value
in case, the method or
attribute is not found.
import statsout

def output(data, format="text"):
output_function = getattr(statsout, "output_%s" % format, statsout.output_text)
return output_function(data)

4.5 Filtering Lists

• Python has powerful capabilities for mapping lists into
other lists, via list comprehensions.
• This can be combined with a ﬁltering mechanism, where
some elements in the list are mapped while others are
skipped entirely.

Filtering syntax:
[mapping-expression for element in source-list if ﬁlter-expression]

Any element for which the ﬁlter expression
evaluates true will be included in the mapping.

All other elements are ignored, so they are never
put through the mapping expression and
are not included in the output list.

Example

• >>> li = [“aaa”, “aa”, “a”, “b”]
• >>> [elem for elem in li if len(elem) == 1]

Ask in class..

methodList = [method for method in dir(object) if callable(getattr(object, method))]

interpretation
please?!?

4.6 The Peculiar Nature
of and and or

• In Python, and and or perform boolean logic as you
would expect, but they do not return boolean values;
instead, they return one of the actual values
they are comparing.

and

• If all values are true, the last value is returned.
• If not all values are true, then it returns the ﬁrst false
value.
• >>> “a” and “b”
'b'
• >>> “” and “b”
''

or

• if any value is true, that value is returned immediately.
• if all values are false, then the last value is returned.

4.7 Using lambda functions

• Python supports an interesting syntax that lets you
deﬁne one-line mini-functions on the ﬂy.
• Borrowed from Lisp, these so-called lambda functions
can be used anywhere a function is required.
• The entire function can only be one expression.
• A lambda is just an in-line function.

Example

• >>> def f(x):
return x*2
• >>> f(3)
• >>> g = lambda x: x*2
• >>> g(3)
• >>> (lambda x: x*2)(3)

Remember:

• To generalize, a lambda function is a function that takes
any number of arguments (including optional arguments)
and returns the value of a single expression.
• Don't try to squeeze too much into a lambda function; if
you need something more complex, deﬁne a normal
function instead and make it as long as you want.
• Use them in places where you want to encapsulate
speciﬁc, non-reusable code without littering your code
with a lot of little one-line functions.

Programming with Python - Week 3

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