Introduction to Python

Interpreter
To start the Python interpreter, type python:

$ python Python 1.5.2 (#1, May 28 1999, 14:49:17) [GCC 2.7.2.3] on linux2 Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam >>>

A simple command:

>>> print "Hello World" Hello World >>>

To leave the interpreter, type Ctrl-D (on Linux) or Ctrl-Z (Windows) or:

>>> import sys >>> sys.exit() $

or:

$ python Python 1.5.2 (#1, May 28 1999, 14:49:17) [GCC 2.7.2.3] on linux2 Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam >>> raise SystemExit $

Note: The examples above are in the interpreter (>>> denotes the prompt); some are as if they were run from a file.

Program files
You can also run Python programs from a file. The usual extension is .py

If we create a file called helloworld.py:

#!/usr/bin/env python print "Hello World"

we can run it with the command:

$ python helloworld.py Hello World $

Language

Statements end in newlines, not with semicolons (;), for example). So the following is a complete command:

>>>print "hello" hello

Blocks are indicated by indentation following a statement ending in a colon (:) as in:

>>> name = "Larry" >>> if name == "George": ... print "Hi George" ... else: ... print "Who are you?" ... print "You're not George!" ... Who are you? You're not George! >>>

The … in this example shows that the block continues until the blank line.

Comments start with # and continue to the end of the line:

print "Tell me." # This is a comment

Identifiers
Identifers are used to name variables, methods, functions or modules. They must start with a non-numeric character, may contain letters, numbers, and underscore (_). Identifiers are case sensitive.

B = 4 b = 2 if b != B: print 'small b is not equal to large B'

Reserved words (native to Python):

and elif global or assert else if pass break except import print class exec in raise continue finally is return def for lambda try del from not while

Identifers with leading __ (double underscore) often have special meanings. Names of variables or functions with leading and trailing __ are used for built-in symbols:

__name__ the name of the function __doc__ the doc string __init__() what we do on startup

Types

• Numeric
  Numeric types are: integer, long integer, floating-point, and complex.

  >>> x = 4 >>> int (x) 4 >>> long(x) 4L >>> float(x) 4.0 >>> complex (4, .2) (4+0.2j) >>>

• String literals
  Enclosing strings in pairs of quotes (‘, ” ,”"”). Two strings together without an operator joins the adjacent strings:

  >>> print "Hi" "there" Hithere >>>

  Backslash (\) escapes special characters except in raw strings:

  >>> print r'n\' # not backslash escaped n\

  Raw strings are often used for regular expressions, because they have quite a few backslashes in them:

  '\[foo\]' r'[foo]' # these are the same

  The single quote (‘) and double quote (“) are equivalent:

  "Don't enclose strings with ' in 's" 'Without escaping the ''

• [ ] Lists
  Integer indexed arrays start at 0:

  >>> months = ["January", "February"] >>> print months[0] January >>> months.append ("March") >>> print months ['January', 'February', 'March']

  Colon (:) is the slicing operator. It lets you work with a portion of the list. The second argument to the slice is non-inclusive ( 1:2 is the second and up to — but not including — the third item in the list):

  >>> print months[1:2] ['February']

  Plus (+) is the concatenation operator:

  >>> print months+months ['January', 'February', 'March', 'January', 'February', 'March']

  Lists can contain any kind of Python object and can be nested:

  >>> months.append (months) >>> print months ['January', 'February', 'March', ['January', 'February', 'March' ]] >>> months.append(1) ['January', 'February', 'March', ['January', 'February', 'March' ], 1]

• () TuplesTuples are the same as lists, but you can’t modify them after you create them. They are usually used as keys into dictionaries.
• {} DictionariesDictionaries are associative arrays or hashes, indexed by a key. A key can be any python object, but it is usually a tuple:
  

  >>> mydict = {"height" : "average", ... "skill" : "intermediate", ... "salary" : 1000 } >>> print mydict {'height': 'average', 'skill': 'intermediate', 'salary': 1000} >>> print mydict["skill"] intermediate >>> mydict[0] = 'Foo' >>> mydict [(5, 6)] = 'Coordinate 5, 6'

  You can use different sorts of Python objects as keys, such as strings and tuples. Lists and dictionaries cannot be used as keys.

Conditionals

>>> q = 4 >>> h = 5 >>> if q

Boolean keywords are: “or,” “and,” “not”

Operators are:

== >=

File Handling

• Read a file
  

  >>> fh = open("helloworld.py") >>> for line in fp.readlines() # read a line (readline method applied to fh) ... print line, # print line, adding newline at end ... #!/usr/bin/python print "Hello World" >>> fh.close()

• Write a file
  

  $ python Python 1.5.2 (#1, May 28 1999, 14:49:17) [GCC 2.7.2.3] on linux2 Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam >>> fh = open("out.txt", "w") >>> fh.write ("we're writing...n") >>> fh.close() >>> # I typed ^D to exit $ cat out.txt we're writing... $

Loops
For is used to iterate over the members of a sequence. It can work with any sequence type (lists, tuples, dictionaries):

>>> for x in range(1,5): ... print x ... 1 2 3 4

Range creates a sequence described by ([start,] end [,step]), where the start and step are optional. Start is assumed to be 0 and step is assumed to be 1.

Functions

Functions are named using the def keyword:

>>> def myfunc(a,b): ... sum = a + b ... return sum ... >>> myfunc (5,6) 11

When you create functions, you can give them default parameters:

>>> def myfunc(a=4,b=6): ... sum = a + b ... return sum ... >>> myfunc() # we use both defaults 10

You can give functions keyword parameters, so the order of parameters doesn’t matter:

>>> myfunc(b=4) # a defaults to 4, we override the value of b 8

Classes

Classes contain collections of methods. Each method refers to the object as the first argument (self) as follows. (See Evelyn’s previous column, Python 101: Testing your code , for a complete discussion.)

class PenguinPen: def __init__(self): self.penguinCount = 0 def add (self, number = 1): """ Add one or more penguins to the pen The default number of penguins to add is 1 """ self.penguinCount = self.penguinCount + number def remove (self, number = 1): """ Remove one or more penguins from the pen """ self.penguinCount = self.penguinCount - number def population (self): """ How many penguins in the pen? """ return self.penguinCount

Exceptions

When an error occurs, an exception is thrown, and a traceback is displayed. I created a file called except.py and it contains:

#!/usr/bin/python print a

If I run it, it returns:

sylvia:/tmp$ python except.py Traceback (innermost last): File "except.py", line 2, in ? print a NameError: a

If you catch the exception, you can handle it:

>>> try: fh=open("new.txt", "r") ... except IOError, e: ... print e ... [Errno 2] No such file or directory: 'new.txt'

You can signal your own exception with the raise statement:

>>> raise MyException

You can also use an exception to quit the interpreter:

>>> raise SystemExit

Modules

A module is collection of methods in a file ending in .py. The name of the file determines the name of the module in most cases:

I created a file called mymodule.py containing:

def one(a): print "in one" def two (c): print "in two"

To use a module, you import it. dir() lists the contents of a module:

>>> import mymodule
>>> dir(mymodule)
['__builtins__', '__doc__', '__file__', '__name__', 'one', 'two']
>>> one
>>> mymodule.one(2)
in one

This text is stolen directly from here

The other Good introduction to Python can be found at this link