Deborah R. Fowler
Python Notes
Updated on April 27 2013 click here for Python Resources
Continuing concepts on Python (for introductory concepts see Turtle Graphics section)
Note: This page was created specifically for VSFX 705 Spring 2013.
So far we have covered variable, truth
statements (selection), looping and functions. Next we'll look
at lists, file i/o and finally oop.
These examples are taken from various web sources listed on the
Python Resources page and are used only for in-class exercises.
Console I/O
name = raw_input("What is your
name?") #
raw_input - input a string from the keyboard
print "Hi " + name + "! How are you?"
#
strings are concatenated with +
print "My name is ", name
# can also use , to print beside
number = input("Input a number ")
# input is used to input
numeric values
Exercises to try: (answer key here)
- Write a script that asks a user for the temperature in Fahrenheit and prints the temperature in Celsius. Given Celsius = (Fahrenheit -32) * 5/9
- Write a currency conversion script
Strings
Delimited by double quotes “ “ Special characters \n is a newline \t is a tab.To print them use a backslash. Multi-line print is enclose in three double quotes “””…”””.
print "To print a newline use \\n"
print "So for \nexample"
print "\t we can indent too"
print """
Many
Many
Many
lines
"""
Exercise to try:
- Write a python script to print
\ | /
0 0
-
\____/
String operators
a = b + c concatenate
a = b * c b repeated c times
a[0] first character of a
len(a)
number of characters in a
Exercise
to try:
Write a program that prints a word in a rectangle using + and *
ie.
hello hello
hello hello
hello
hello
hello
hello
hello
hello hello
hello
Lists
| Examples |
Explanations |
| cards = [“ace”, “king”, “queen”] |
defines a list |
| cards[0] is “ace” | elements are accessed via subscripts |
| cards[0] = “spot” | You can assign to a list |
| cards.append(“more”) | You can append to the end of a list |
| cards = [“well isn’t this fancy”,”yes”] + cards | You can add to the beginning |
| del cards[3] | delete an element by index number |
| cards.remove(“queen”) | delete by value using remove |
|
cards.insert(1,”test”) |
insert an element as this position |
| len(cards) | length |
| max(cards) |
max and min |
| newsomething = cards[:] |
a new copy |
| cards.reverse() |
reverse() and sort() |
Can use for statements with lists (in fact you have
used this)
for card in cards:
print card;
Dictionaries - lists whose indices are numbers and strings
Try hangman (this from http://inventwithpython.com/IYOCGwP_book1.pdf)
File
I/O
file
= open(“outputFile.txt”,”a”)
# open for appending
file
= open("someAsciiFile.txt","r")
# open for reading
file = open(“outputFile.txt”,”w”) # open for writing
file.close()
# close the file
text = file.readlines() # read all lines into a list called text
line = file.readline() # read next line into a string called line
file.writelines(text) # write list to file
file.write(line)
# write
one
line to file
For example:
file = open("someAsciiFile.txt","r")
text = file.readlines()
file.close()
for line in text:
print line
If your file is huge, use the following instead
#!/usr/bin/env python
#
# Program to read and print a file
#
file = open("alice.txt","r")
line = file.readline()
while line:
print line,
line = file.readline()
print
file.close()
Adding
some error checking:
#!/usr/bin/env
python
#
# Program to read and print a file
import sys
try:
file = open("sample.txt","r")
except IOError:
print "Could not open file"
sys.exit()
text = file.readlines()
file.close()
for line in text:
print line,
print
OOP
- see next set of notes