Python Basics

Check that Python is Installed

python

Python 1.5.1 (#1, Sep  3 1998, 22:51:17)  [GCC 2.7.2.3] on linux-i386
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>>

If that message does not appear, do what it takes to get python installed and accessible.

Hello World

print "Hello World"

Be absolutely sure not to have any spaces to the left of the word "print". Python uses indentation sensitive syntax, and will fail if there are any spaces or tabs to the left of the print statement.

Run it with this command:

python hello.py

Make it an Executable

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

The top line assumes that your Python executable is in /usr/bin. If it's somewhere else, substitute that path.

Now make the file executable with this command:

chmod a+x hello.py

./hello.py

Notice that a semicolon is NOT required at the end of the line. Semicolons are necessary ONLY to separate multiple commands on the same line. Otherwise, the end of the line ends the command.

Add a Variable

#!/usr/bin/python
myname = "Steve Litt"
print "Hello World"
print myname

This should print a line saying "Hello World", followed by a line saying "Steve Litt" to the screen. If not, troubleshoot.

Notice these things:

• It prints newlines automatically. If you wish to suppress the newline printing, end the print line with a comma, in which case it prints an extra space but no newline.
• To avoid the space at the end of the print, do string concatenation.

Add Comments

#!/usr/bin/python
myname = "Steve Litt"
#This is a comment
#   So is this
   #And this is a comment
print "Hello World"  #This is an inline comment
print myname

Note the following:

• Comments begin with a pound sign, hash symbol, #
• Comments alone on their own lines are not subject to indentation sensitive syntax.
• Inline comments are preceeded to a pound sign, and are to the right of an executable.

Simple String Concatenation

#!/usr/bin/python
longname = "Anderson"
print longname
longname = longname + "berg"
print longname

This program should print a line saying "Anderson", followed by another line saying "Andersonberg". If not, troubleshoot.

Python can do incredible things with strings, including all the regular expression activities as seen in Perl. These will be presented later in this tutorial.

Create a Loop and Handle Indentation

#!/usr/bin/python
ss = 1
while ss < 10:
   print "Inside loop, subscript is", ss
   ss = ss + 1
print "Finished looping."

This should print out nine lines stating the subscript, and a final line saying "Finished Looping". If anything goes wrong, make sure the while statement ends with a colon and check your indentation.

Test at Top

Indentation Sensitive Syntax NOTE THIS

Everything intended to be subservient to the while statement (in other words, all the statements to be looped through), should be indented a uniform amount to the right of the while statement. That indentation can be either hard tabs, or (my preference), a certain amount of spaces. For the sake of future maintainers of your code, use one or the other consistently.

As a generality, whenever a Python statement ends in a colon (like the while statement above), it's expected to have subservient statements, and those subservient statements must be consistently indented relative the statement with the colon.

Advantages of Indation Sensitive Syntax

if (i < 10)
  {
  printf("Less than 10\n");
  i = 20;
  }

or

if (i < 10)
{
  printf("Less than 10\n");
  i = 20;
}

or

if (i < 10){
  printf("Less than 10\n");
  i = 20;
  }

or

if (i < 10){
  printf("Less than 10\n");
  i = 20;
}

or even

if (i < 10)  {
                      printf("Less than 10\n");
i = 20;
                 }

if (i < 10):
  printf("Less than 10\n")
  i = 20

I'm not saying indentation sensitive syntax is perfect. It can be maddening to fail because of an extra space. But in these days of inadequate documentation, the simplicity and self-documentation of Python syntax can be welcome.

Insert an If Statement Inside the Loop

#!/usr/bin/python
ss = 1
while ss < 10:
   if ss % 3 == 0:
      print "Inside loop, subscript is", ss,
      print "Divisible by three"
   elif ss % 2 == 0:
      print "Inside loop, subscript is", ss,
      print "Divisible by two"
   else:
      print "Inside loop, subscript is", ss,
      print "Divisible neither by three nor two"
   ss = ss + 1
print "Finished looping."

This should put out the following:

Inside loop, subscript is 1 Divisible neither by three nor two
Inside loop, subscript is 2 Divisible by two
Inside loop, subscript is 3 Divisible by three
Inside loop, subscript is 4 Divisible by two
Inside loop, subscript is 5 Divisible neither by three nor two
Inside loop, subscript is 6 Divisible by three
Inside loop, subscript is 7 Divisible neither by three nor two
Inside loop, subscript is 8 Divisible by two
Inside loop, subscript is 9 Divisible by three
Finished looping.

Note the following:

• If statements can be followed by any number of elif statements (else if), and one else statement. If, elif and else all have colons, and statements to all three are indented relative to the if, elif or else.
• Python's indentation and lack of block specifiers makes for much more readable code.

Use the Pass Statement

#!/usr/bin/python
month = 3    #var in if statement must have been declared
if month == 1:
   pass
elif month < 6:
   print "early"
else:
   print "late"

Here January does nothing, months Feb thru May print "early", and months June thru December print "late". This is not a particularly good example, but this is how the pass statement is used.

Use the Break (but not the Continue) Statements

On the other hand, break statements can sometimes improve readability and even maintainability. This is especially true when two different conditions can stop the action. Create this break.py program, make it executable and execute it. Note that the strategic placement of the break statement prevents an extra comma after the last number. Note that the while 1 would go on forever if not for the break statement.

#!/usr/bin/python ss = 1 while 1:    print ss,    if(ss == 9):       break;    print ",",    ss = ss + 1 print "."

The output of this program should be:

1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 .

Lists

Hello World

#!/usr/bin/python
seasons = ["Winter","Spring","Summer","Fall"]
print seasons

['Winter', 'Spring', 'Summer', 'Fall']

What you have done here is created a list. Lists are mutable (changable), both in terms of changing individual members and in terms of adding, deleting or rearranging members.
 

Accessing List Members

#!/usr/bin/python
seasons = ["Winter","Spring","Summer","Fall"]
print len(seasons)              #there are 4 seasons
print seasons[0]                #element 0, the 1st element
print seasons[2]                #element 1
print seasons[len(seasons)-1]   #last element
print seasons[-1]               #last element
print seasons[0:2]              #elements 0 and 1, (1st & 2nd)
print seasons[1:3]              #elements 1 and 2
print seasons[1:9]              #1 thru last, overrange not error
print seasons[-1:9999]          #last element
print seasons[-1:len(seasons)]  #last element
print seasons[-3:-2]            #3rd &2nd to last elements


#  print seasons[len(seasons)]   would produce an error, as
#     the last element is seasons[3].                         

• len(listname) returns the number of elements in the list. Since the list is zero based, the number of elements equals one plus the index of the last number.
• listname[x] represents the xth member of the list, where the first member is listname[0].
• listname[s:e] is called "slicing" the list, and represents a subset list, of all listname elements with indexes s <= x < e. In other words, starting with the s element of the original list, and containing all members up to, but not including, the e member. Please remember all these subscripts are zero based. If the ending number is too big, it simply delivers the remainder of the list including and after element s.
• listname[-s:-e] is the a subset list starting with the member whose index len(listname) - s, and ends BEFORE len(listname) - e.
• Other combinations are possible. Experiment.
• Accessing a single element beyond the upper limit produces an error.

Looking Up List Members by Value

#!/usr/bin/python
seasons = ["Winter","Spring","Summer","Fall"]
n = seasons.index("Summer")
print n
print seasons[n]

Use this with care. If the argument to index is not a value in the list, the program fails to run. also, index pulls up only the first element with that value. To be sure all such elements have been deleted, use listname.count(string) to get a count of the number of times it appears, then run through a loop to get them all. Note also that this prevents a non-occurring string from bombing the program.

Inserting List Members

#!/usr/bin/python
seasons = []
seasons.insert(len(seasons), "Spring") #append to empty list
seasons.insert(0, "Winter")            #insert at beginning of list
seasons.insert(len(seasons), "Fall")   #append to end of list
print seasons
seasons.insert(2, "Summer")            #insert before seasons[2], "Fall"
print seasons

In this exercise, you inserted elements at the beginning, at the end, and in the middle.

Appending List Members

#!/usr/bin/python
seasons = []
seasons.append("Winter")                  #append element
print seasons
seasons = seasons + ["Spring", "Summer"]  #append a list to a list
print seasons                             
seasons.append("Fall")                    #append element
print seasons

In this exercise you used the append command to append elements, and you used the plus (+) operator to append an entire list.

Deleting a List Member

#!/usr/bin/python
seasons = ["Winter","Spring","Summer","Fall"]
print seasons
del seasons[2]
print seasons
seasons = ["Winter","Spring","Summer","Fall"]
print seasons
del seasons[1:3]
print seasons

Add a For Loop

#!/usr/bin/python
seasons = ["Winter","Spring","Summer","Fall"]
for x in seasons:
   print x, "season."

The output here should be:

Winter season.
Spring season.
Summer season.
Fall season.

Practical Exercise: A Data Generator

#!/usr/bin/python

lastnames = ["Albright","Barclay","Chavez","Dugan","Eng"]
firstnames = ["Allen","Betty","Charles","Donna","Edward"]
empno = 1001
outstring = ""

for first in firstnames:
   for last in lastnames:
      print "%04d,%s,%s" % (empno, last, first) #comma delimeted
      empno = empno + 1

Note the following:

• Nested for loops
• Enhanced print statement very much like C's printf(). Note the use of the percent sign between the format string and the list of args, and the fact that the args are in parens.
• Slightly modified, this program is actually useful for producing test data. It's 11 lines, counting blank lines.
• With a name per alphabet letter, and a list of middle names (another nest in the loop), it can output 26*26*26 different names, for a total of 17,576 records.