Setting the PYTHONPATH
• PYTHONPATH is where Python looks for modules it is
told to import
• List of paths
• Add new path to the end with:
setenv PYTHONPATH ${PYTHONPATH}:/users/chili/Pythonmodules/
export PYTHONPATH=${PYTHONPATH}:/users/chili/Pythonmodules/
(C shell and Bash, respectively)
Permanently adding path to PYTHONPATH
• Create/load file called
~/.daimi-setup/tcsh/tcshrc.d/path.rc
• Add the magic line to it
• To later add more paths:
setenv PYTHONPATH
${PYTHONPATH}:/users/chili/Pythonmodules/:/users/chili
/Pythonmodules/RealGoodies/
How arguments are transferred in a function call
function
object
global
namespace
def f(a):
q = 7
z = 13
f
z
13
local
namespace
f(z)
a
q
7
How arguments are transferred in a function call
function
object
global
namespace
def f(a):
q = 7
a = 8
z = 13
f
z
13
local
namespace
f(z)
print z
a
q
8
7
Retrieving a variable’s
value
import math
def f(a):
q = 7
?
print y
built_in
global
raw_input,
int,
float,
..
__name__ = ‘__main__’,
math,
f,
x = 9,
local
z = 13,
..
a = 13,
q = 7
x = 9
z = 13
f(z)
Two x’es
built_in
import math
global
def f(a):
x = 7
print x
raw_input,
int,
float,
..
x = 9
z = 13
__name__ = ‘__main__’,
math,
f,
x = 9,
local
z = 13,
..
a = 13,
x = 7
global x
f(z)
local x
print x
Value of x ..? 9.
Keyword global
built_in
import math
global
def f(a):
global x
x = 7
range,
raw_input,
int,
float,
..
__name__ = ‘__main__’,
math,
f,
x = ..,
local
z = 13,
..
a = 13
x = 9
z = 13
global x
f(z)
print x
Value of x..? 7!
no x
here
Scope of a variable
The region of the program where the variable is accessible
Using a variable outside its scope:
def area(r):
 a = area(1.0)
pi = 3.15
return pi*r*r*
def area(r):
pi = 3.15
a = area(1.0)
 print pi
return pi*r*r*
Example: a function for computing
the factorial of n
1!
2!
7!
n!
=
=
=
=
1
2*1
7*6*5*4*3*2*1
n*((n-1)!)
I.e. the factorial can be defined in terms of itself
Algorithm:
If n < 2: return n
Otherwise return n*((n-1)!
demo
A recursive function
factorial.py
Documentation
string. Printed by
help() if called on
this function
This code is only executed if
the file is python’ed directly,
not if it is imported.
demo
Executing file directly:
threonine:~...ProgramExamples% python factorial.py
n: 10
3628800
n: 4
24
n: -1
threonine:~...ProgramExamples%
Importing file:
threonine:~...ExamplePrograms% python
Python 2.3.4 (#1, Feb 2 2005, 12:11:53)
[GCC 3.4.2 20041017 (Red Hat 3.4.2-6.fc3] on linux2
Type "help", "copyright", "credits" or "license" for
more information.
>>>
>>> from factorial import recursive_factorial as f
>>> f(5)
>>> 120
Default Arguments
A function may sometimes be called repeatedly
with the same values
– Default arguments can be set in function definition
• Must appear to the right of undefaulted parameters:
def myfunction( a, b = 2, c = 3 ):
• Given values are ‘filled in from the left’ (resolves ambiguity)
myfunction(6, 3) # set a and b, use default for c
myfunction(6)
# set a, use default for b and c
– A default value can also be overridden:
myfunction(6, 3, 7) # overrides default c value
def boxVolume( length = 3, width = 2, height = 1 ):
return length * width * height
All default values used
print "The default box volume is:", boxVolume()
print "\nThe volume of a box with length 10,"
print "width 2 and height 1 is:", boxVolume( 10 )
The 10 will replace the 3
and the other default
values will be used
Here two values are
print "\nThe volume of a box with length 10,"
print "width 5 and height 1 is:", boxVolume( 10, 5 ) sent replacing the two
left-most default values
print "\nThe volume of a box with length 10,"
print "width 5 and height 2 is:", boxVolume( 10, 5, 2 )
No default
values used
The default box volume is: 6
The volume of a box with length 10, width 2 and height 1 is: 20
The volume of a box with length 10, width 5 and height 1 is: 50
The volume of a box with length 10, width 5 and height 2 is: 100
Keyword Arguments
– You can use argument names as keywords:
– Allows arguments to be passed in any order:
def myfunction( a = 2, b = 3 ):
..
myfunction ( b = 5, a = 9 )
Some of the
parameters have
default values
Sets first
argument, uses
the defaults for
all the others
name can be used as keyword as
well, even though it doesn’t have
a default value
Personal data: George, 53, president (US)
Personal data: Tony, 53, prime minister (GB)
Personal data: Ronald, 92, president (US)
The parameters
are given new
values except
age which uses
the default.
Values need not
be entered in
order.
How not to use keyword arguments
Keyword arguments must appear to the right of any
other arguments in a function call. Otherwise
ambiguity may arise.
>>>
>>> def test( name, age = 10, town = “Canberra” ):
...
pass
...
Is “Alice” the value for
>>> test( age = 30, “Alice" )
name or town?
SyntaxError: non-keyword arg after keyword arg
>>>
>>>
No value given for name which
>>> test( age = 30 )
Traceback (most recent call last):
doesn’t have a default value
File "<stdin>", line 1, in ?
TypeError: test() takes at least 1 non-keyword argument (0
given)
Rule of thumb: arguments determined by position must come first
On to the exercises..