If a line in a program contains the symbol #, everything following on the same line is considered as a comment:

# This is a comment of the following statement
a = 3  # ... which might get a further comment here  

A backslash \ at the end of the line marks the next line as a continuation line, that is, explicit line joining. If the line ends before all the parentheses are closed, the following line will automatically be recognized as a continuation line, that is, implicit line joining.

A number may be an integer, a real number, or a complex number. The usual operations are:

Here is an example:

2 ** (2 + 2) # 16
1j ** 2 # -1
1. + 3.0j

Strings are sequences of characters, enclosed by simple or double quotes:

'valid string'
"string with double quotes"
"you shouldn't forget comments"
'these are double quotes: ".." '

You can also use triple quotes for strings that have multiple lines:

"""This is
 a long,
 long string"""

A variable is a reference to an object. An object may have several references. One uses the assignment operator = to assign a value to a variable:

x = [3, 4] # a list object is created
y = x # this object now has two labels: x and y
del x # we delete one of the labels
del y # both labels are removed: the object is deleted

The value of a variable can be displayed by the print function:

x = [3, 4] # a list object is created
print(x)

Lists are a very useful construction and one of the basic types in Python. A Python list is an ordered list of objects enclosed by square brackets. One can access the elements of a list using zero-based indexes inside square brackets:

L1 = [5, 6]
L1[0] # 5
L1[1] # 6
L1[2] # raises IndexError
L2 = ['a', 1, [3, 4]]
L2[0] # 'a'
L2[2][0] # 3
L2[-1] # last element: [3,4]
L2[-2] # second to last: 1

Indexing of the elements starts at zero. One can put objects of any type inside a list, even other lists. Some basic list functions are as follows:

      print(list(range(5))) # returns [0, 1, 2, 3, 4]

      len(['a', 1, 2, 34]) # returns 4

      L = ['a', 'b', 'c']
      L[-1] # 'c'
      L.append('d')
      L # L is now ['a', 'b', 'c', 'd']
      L[-1] # 'd'

A Boolean expression is an expression that may have the value True or False. Some common operators that yield conditional expressions are as follow:

One combines different Boolean values with or and and. The keyword not , gives the logical negation of the expression that follows. Comparisons can be chained so that, for example, x < y < z is equivalent to x < y and y < z. The difference is that y is only evaluated once in the first example. In both cases, z is not evaluated at all when the first condition, x < y, evaluates to False:

2 >= 4  # False
2 < 3 < 4 # True
2 < 3 and 3 < 2 # False
2 != 3 < 4 or False # True
2 <= 2 and 2 >= 2 # True
not 2 == 3 # True
not False or True and False # True!

One typical use of a for loop is to repeat a certain task a fixed number of times:

n = 30
for iteration in range(n):
    do_something # this gets executed n times

The for statement has two important keywords: break and else. break quits the for loop even if the list we are iterating is not exhausted:

for x in x_values:
   if x > threshold:
     break
   print(x)

The finalizing else checks whether the for loop was broken with the break keyword. If it was not broken, the block following the else keyword is executed:

for x in x_values:
    if x > threshold:
       break
else:
    print("all the x are below the threshold")

Often one collects functions in a script. This creates a module with additional Python functionality. To demonstrate this, we create a module by collecting functions in a single file, for example smartfunctions.py:

def f(x):
    return 2*x + 1
def g(x):
    return x**2 + 4*x - 5
def h(x):
    return 1/f(x)

Again, the command exec(open('smartfunctions.py').read()) makes these functions available to your IPython shell (note that there is also the IPython magic function run). In Python terminology, one says that they are put into the actual namespace.

Alternatively, the modules  can be imported by the command import. It creates a named namespace. The command from puts the functions into the general namespace:

import smartfunctions
print(smartfunctions.f(2))      # 5

from smartfunctions import g    #import just this function
print(g(1)) # 0
  
from smartfunctions import *    #import all
print(h(2)*f(2))                # 1.0