Transcript Introduction to Python - College of Computing & Informatics

Introduction to Python
What is Python?
• Interpreted object oriented high level
programming language
– No compiling or linking neccesary
• Extensible: add new functions or modules to
the interpreter
Benefits
• Offers more structure and support for large
programs than shell and batch files
• Modules – can be reused
• No compilation saves development time
• Easy to read syntax
The Interpreter
• Similar to UNIX
– Reads and executes commands interactively
– When called with a file name argument, it reads
and executes a script from that file.
• Invoking is simple:
– python
or
– python -c command [arg] ...
• Script name and additional arguments are
passed to sys.argv
• When commands are read from a tty, the
interpreter is said to be in interactive mode.
– Primary Prompt: >>>
– Secondary Prompt: ...
• Continuation lines are needed when entering a multiline construct. As an example, take a look at this if
statement:
Example:
>>> the_world_is_flat = 1
>>> if the_world_is_flat:
...
print "Be careful not to fall off!"
...
Be careful not to fall off!
Using Python as a Calculator
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>>> 2+2
4
>>> # This is a comment
... 2+2
4
>>> 2+2 # and a comment on the same line
as code
4
>>> (50-5*6)/4
5.0
>>> 8/5 # Fractions aren’t lost when
dividing integers
1.6
Using Variables
>>> width = 20
>>> height = 5*9
>>> width * height
900
>>>
>>>
0
>>>
0
>>>
0
x = y = z = 0 # Zero x, y and z
x
y
z
Using Variables
>>> # try to access an undefined variable
... n
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name ’n’ is not defined
>>> # last printed expression is stored as _
>>> price * tax
12.5625
>>> price + _
113.0625
Strings
>>> ’spam eggs’
’spam eggs’
>>> ’doesn\’t’
"doesn’t"
>>> "doesn’t“
"doesn’t“
>>> ’"Yes," he said.’
’"Yes," he said.’
>>> "\"Yes,\" he said."
’"Yes," he said.’
>>> ’"Isn\’t," she said.’
’"Isn\’t," she said.’
New lines
hello = "This is a rather long string
containing\n\
several lines of text just as you would do
in C.\n\
Note that whitespace at the beginning of the
line is\
significant."
print(hello)
This is a rather long string containing
several lines of text just as you would do
in C.
Note that whitespace at the beginning of the
line is significant.
Manipulating Strings
>>> word = ’Help’ + ’A’
>>> word
’HelpA’
>>> ’<’ + word*5 + ’>’
’<HelpAHelpAHelpAHelpAHelpA>’
>>> ’str’ ’ing’
’string’
>>> ’str’.strip() + ’ing’
’string’
Subscripts
>>> word[4]
’A’
>>> word[0:2]
’He’
>>> word[2:4]
’lp’
>>> word[:2] # The first two characters
’He’
>>> word[2:] # Everything except the first
two characters
’lpA’
More Slice Notation
>>> word[0] = ’x’
Traceback (most recent call last):
File "<stdin>", line 1, in ?
>>> ’x’ + word[1:]
’xelpA’
>>> ’Splat’ + word[4]
’SplatA’
>>> word[1:100]
’elpA’
>>> word[10:]
’’
>>> word[2:1]
’’
More Slice Notation
>>> word[-1] # The last character
’A’
>>> word[-2] # The last-but-one character
’p’
>>> word[-2:] # The last two characters
’pA’
>>> word[:-2] # Everything except the last
two characters
’Hel’
Lists
• Most versatile compound data type.
• List items may be different types.
>>> a = [’spam’, ’eggs’, 100, 1234]
>>> a
[’spam’, ’eggs’, 100, 1234]
Lists Indices
>>> a[0]
’spam’
>>> a[3]
1234
>>> a[-2]
100
>>> a[1:-1]
[’eggs’, 100]
>>> a[:2] + [’bacon’, 2*2]
[’spam’, ’eggs’, ’bacon’, 4]
>>> 3*a[:3] + [’Boo!’]
[’spam’, ’eggs’, 100, ’spam’, ’eggs’, 100,
’spam’, ’eggs’, 100, ’Boo!’]
Programming
>>> # Fibonacci series:
... # the sum of two elements defines the
next
... a, b = 0, 1
>>> while b < 10:
... print(b)
... a, b = b, a+b
...
1
1
2
3
5
8
if Statements
>>> x = int(input("Please enter an
integer: "))
Please enter an integer: 42
>>> if x < 0:
... x = 0
... print(’Negative changed to zero’)
... elif x == 0:
... print(’Zero’)
... elif x == 1:
... print(’Single’)
... else:
... print(’More’)
...
More
for Statements
>>> # Measure some strings:
... a = [’cat’, ’window’, ’for’]
>>> for x in a:
... print(x, len(x))
...
cat 3
window 6
for 3
Using range() function
>>> for i in range(5):
... print(i)
...
0
1
2
3
4
Using break and else for loops
>>> for n in range(2, 10):
...
for x in range(2, n):
...
if n % x == 0:
...
print(n, ’equals’, x,
’*’, n//x)
...
break
...
else:
...
# loop fell through without
finding a factor
...
print(n, ’is a prime number’)
...
Using pass
The pass statement is used for when a statement is
required but no action.
>>> while True:
... pass # Busy-wait for keyboard
interrupt (Ctrl+C)
...
>>> class MyEmptyClass:
... pass
...
Defining Functions
>>> def fib(n): # write Fibonacci series up to
n
... """Print a Fibonacci series up to n."""
... a, b = 0, 1
... while b < n:
...
print(b, end=’ ’)
...
a, b = b, a+b
... print()
...
>>> # Now call the function we just defined:
... fib(2000)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
1597
Keyword Arguments
-May use keyword arguments of the form:
keyword = value
def parrot(voltage, state=’a stiff’,
action=’voom’):
print("-- This parrot
wouldn’t", action, end=’
’)
print("if you put", voltage,
"volts through it.")
http://docs.python.org/tutorial/index.html