Python 101 - Introduction to Python

Simple Statements

print

The print statement sends output to stdout.

Here are a few examples:

print obj

print obj1, obj2, obj3

print "My name is %s" % name

Notes:

• To print multiple items, separate them with commas. The print statement inserts a blank between objects.

• The print statement automatically appends a newline to output. To print without a newline, add a comma after the last object, or use "sys.stdout", for example:

  print 'Output with no newline',
  
  

  which will append a blank, or:

  import sys
  
  sys.stdout.write("Some output")
  
  

• To re-define the destination of output from the print statement, replace sys.stdout with an instance of a class that supports the write method. For example:

  import sys
  
  
  
  class Writer:
  
      def __init__(self, filename):
  
          self.filename = filename
  
      def write(self, msg):
  
          f = file(self.filename, 'a')
  
          f.write(msg)
  
          f.close()
  
  
  
  sys.stdout = Writer('tmp.log')
  
  print 'Log message #1'
  
  print 'Log message #2'
  
  print 'Log message #3'
  
  

  Download as text (original file name: Examples/python_101_print_sysstdout.py).

More information on the print statement is at http://www.python.org/doc/current/ref/print.html.

Note: Note to Jython users - Jython does not appear to support the file constructor for files. In the above example, replace file with open.

import

The import statement makes a module and its contents available for use.

Here are several forms of the import statement:

import test

Import module test. Refer to x in test with "test.x".

from test import x

Import x from test. Refer to x in test with "x".

from test import *

Import all objects from test. Refer to x in test with "x".

import test as theTest

Import test; make it available as theTest. Refer to object x with "theTest.x".

A few comments about import:

• The import statement also evaluates the code in the imported module.

• But, the code in a module is only evaluated the first time it is imported in a program. So, for example, if a module mymodule.py is imported from two other modules in a program, the statements in mymodule will be evaluated only the first time it is imported.

• If you need even more variety that the import statement offers, see the imp module. Documentation at http://www.python.org/doc/current/lib/module-imp.html. Also see the __import__ built-in function. Documentation at http://www.python.org/doc/current/lib/built-in-funcs.html.

More information on import at http://www.python.org/doc/current/ref/import.html.

assert

Use the assert statement to place error checking statements in you code. Here is an example:

def test(arg1, arg2):

    arg1 = float(arg1)

    arg2 = float(arg2)

    assert arg2 != 0, 'Bad dividend -- arg1: %f arg2: %f' % (arg1, arg2)

    ratio = arg1 / arg2

    print 'ratio:', ratio

When arg2 is zero, running this code will produce something like the following:

Traceback (most recent call last):

  File "tmp.py", line 22, in ?

    main()

  File "tmp.py", line 18, in main

    test(args[0], args[1])

  File "tmp.py", line 8, in test

    assert arg2 != 0, 'Bad dividend -- arg1: %f arg2: %f' % (arg1, arg2)

AssertionError: Bad dividend -- arg1: 2.000000 arg2: 0.000000

A few comments:

• Notice that the trace-back identifies the file and line where the test is made and shows the test itself.

• If you run python with the optimize options (-O and -OO), the assertion test is not performed.

• The second argument to assert is optional.

global

The problem -- Imagine a global variable NAME. If, in a function, the first mention of that variable is "name = NAME", then I'll get the value of the the global variable NAME. But, if, in a function, my first mention of that variable is an assignment to that variable, then I will create a new local variable, and will not refer to the global variable at all. Consider:

NAME = "Peach"



def show_global():

    name = NAME

    print '(show_global) name: %s' % name



def set_global():

    NAME = 'Nectarine'

    name = NAME

    print '(set_global) name: %s' % name



show_global()

set_global()

show_global()

Running this code produces:

(show_global) name: Peach

(set_global) name: Nectarine

(show_global) name: Peach

The set_global modifies a local variable and not the global variable as I might have intended.

The solution -- How can I fix that? Here is how:

NAME = "Peach"



def show_global():

    name = NAME

    print '(show_global) name: %s' % name



def set_global():

    global NAME

    NAME = 'Nectarine'

    name = NAME

    print '(set_global) name: %s' % name



show_global()

set_global()

show_global()

Notice the global statement in function set_global. Running this code does modify the global variable NAME, and produces the following output:

(show_global) name: Peach

(set_global) name: Nectarine

(show_global) name: Nectarine

Comments:

• You can list more than one veriable in the global statement. For example:

  global NAME1, NAME2, NAME3

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