A brief introduction to the Python programming language: variables, operators, lists, conditionals, loops, file input/output.

1. Python
A brief introductory guide

2. A few things about Python
• Python is a widely used general-purpose, highlevel programming language.
• Python’s main design philosophy:
– Highly readable and simple code
– As few lines of code as possible
– Dynamic type system (explain later)
• Python is used by:
– Google
– NASA
– New York Stock Exchange

3. Starting up
“Bare” Python is too general for scientific application, so we’re
going to use the distributives with additional modules for
graphing, statistical analysis, matrices etc.
• (OPTION 1) Download Anaconda distributive:
https://store.continuum.io/cshop/anaconda/
• (OPTION 2) Download Python(x, y) distributive:
http://code.google.com/p/pythonxy/
Install (available on Mac, Windows and Linux platforms)

4. Loading the IDE
IDE is a platform that lets you write code, execute programs and
test them. Both Anaconda and Python(x, y) packages have the
Spyder IDE already installed.
Search for the Spyder in the programs list, run the application.
Alternative: write code in ANY text editor, save it with a “.py”
extension, and run via the console.

5. Run scripts
Write code
See output
Spyder IDE

6. Basics: variables
a = 4 # Integer
b = 5.6 # Float (Dynamic type)
c = "hello" # String
a = "4" # rebound to String
Output variables into the console by writing:
print variable_name

7. Basics: operators
a=1
b=2
print
print
print
print
print
print
a+b #3
a – b # -1
a*b #2
a / b # 0 (careful with that!)
1.0 / 2 # 0.5
b ** 2 # 4 (** is a power operator)

8. Basics: strings
You can access substrings of a string:
string = ‘Hello World’
print string[0] # H
print string [6:] # World
You also can concatenate two strings:
string2 = ‘ Omega’
pring string + string2 # Hello World Omega
To concatenate strings and integers, use str() function

9. Lists
A list is a collection of values, stored in one variable.
Some number is assigned for each item in the list.
list1 = *1, 5, 12, ‘Car’+
print list1[2] # 12 (start of numeration from 0)
print len(list1) # 4 (number of elements)
You can append, delete and replace items in the list:
list1.append(10) # *1, 5, 12, ‘Car’, 10]
list1.insert(0, 2) # [2, 1, 5, 12, ‘Car’, 10+
list1.remove(‘Car’) # [2, 1, 5, 12, 10]

10. Dictionaries
Similar to list, but instead of numeric values, the
identifier of each element is a string:
dic = ,‘Bob’:100, ‘Jake’: 350print dic*‘Jake’+ # 350
dic *‘Henry’+ = 1000
With dictionaries, you can easily check if the specific
value is inside:
print ‘Jake’ in dic # True

11. Conditional statements
If-else-elif are used to check a condition and do
different stuff depending on the outcome:
dic = ,‘Bob’:100, ‘Jake’: 350if ‘Jake’ in dic:
print ‘Jake is in there’
elif ‘Bob’ in dic:
print ‘Bob is in there’ # won’t execute (why?)
else:
pass # do nothing
Very important: you have to do the indentations!

12. For/in Loops
Let’s rewrite the previous example:
dic = ,‘Bob’:100, ‘Jake’: 350for item in dic:
print item + ' is in there‘ # item is an index now
print item + ‘ has $’ + str(dic[item])
Bob is in there
Bob has $100
Jake is in there
Jake has $350

13. While loops
The logic is almost the same:
list = range(1, 101) # generate integers from 1 to 100
count = 0;
while count < len(list):
print list[count]
count += 1 # increment by 1 each time
1
2
…

14. File input/output
Open a file to read from it:
fin = open(‘foo.txt’)
for line in fin:
print line # manipulate each line
fin.close() # important to do after all manipulations
Write into a file:
fout = open(‘foo.txt’, ‘w’) # set the ‘write’ mode
fout.write(‘hello world’)
fout.close()

15. Further info
Codeacademy website:
http://www.codecademy.com/ru/tracks/python
Python official documentation:
http://docs.python.org/2/tutorial/index.html