Python Notes : An Informal Introduction to Python

Using Python as a Calculator

use the console

Numbers

MDAS - multiplication, division, addition, subtraction
division always returns a floating point number
to do floor division and get an integer result use the // operator
to calculate the remainder use the % operator

>>> 17 / 3
5.666666666666667
>>> 17 // 3  # floor division discards the fractional part
5
>>> 17 % 3  # the % operator returns the remainder of the division
2

use the ** operator to calculate powers

>>> 5 ** 2  # 5 squared
25
>>> 2 ** 7  # 2 to the power of 7
128

full support for floating pointmath
operators with mixed type operands convert the integer operand to floating point

>>> 4 * 3.75 - 1
14.0

in interactive mode, the last printed expression is assigned to the read-only variable _

>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06

Text

string represented by type str
can use single or double quotes
to quote a quote escape character is used

>>> 'doesn\'t'  # use \' to escape the single quote...
"doesn't"
>>> "doesn't"  # ...or use double quotes instead
"doesn't"
>>> '"Yes," they said.'
'"Yes," they said.'
>>> "\"Yes,\" they said."
'"Yes," they said.'
>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'

use print() function for readable output

>>> s = 'First line.\nSecond line.'  # \n means newline
>>> s  # without print(), special characters are included in the string
'First line.\nSecond line.'
>>> print(s)  # with print(), special characters are interpreted, so \n produces new line
First line.
Second line.

can print raw strings without escape chars using r before the string

>>> print('C:\some\name')  # here \n means newline!
C:\some
ame
>>> print(r'C:\some\name')  # note the r before the quote
C:\some\name

a raw string may not end in an odd number of \ characters
string literals can span multiple lines
one way is to use triple-quotes: """...""" or '''...'''
end of lines are automatically included in the string
prevent end of line by adding a \ at the end of the line

>>> print("""\
...Usage: thingy [OPTIONS]
...     -h                        Display this usage message
...     -H hostname               Hostname to connect to
...""")
Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to

strings can be concatenated with the + operator, and repeated with *

>>> # 3 times 'un', followed by 'ium'
>>> 3 * 'un' + 'ium'
'unununium'

two or more string literals next to each other are automatically concatenated

>>> 'Py' 'thon'
'Python'

useful when breaking long strings

>>> text = ('Put several strings within parentheses '
 ...       'to have them joined together.')
>>> text
'Put several strings within parentheses to have them joined together.'

only works with two literals though, not with variables or expressions
concatenate variables or a variable and a literal, use +

>>> prefix + 'thon'
'Python'

strings can be indexed

>>> word = 'Python'
>>> word[0]  # character in position 0
'P'
>>> word[5]  # character in position 5
'n'

indices can be negative
count starts from the right
negative indices start at -1

>>> word[-1]  # last character
'n'
>>> word[-2]  # second-last character
'o'
>>> word[-6]  # note case is maintained
'P'

slicing for substrings can be done

>>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
'tho'

slicing has defaults

an omitted first index defaults to zero
an omitted second index defaults to the size of the string being sliced

>>> word[:2] + word[2:]
'Python'
>>> word[:4] + word[4:]
'Python'

for non-negative indices the length of a slice is the difference of the indices, if both are within bounds
the length of word[1:3] is 2
using an index that is too large will result in an error

>>> word[42]  # the word only has 6 characters
Traceback (most recent call last):
  File "", line 1, in 
IndexError: string index out of range

out of range slicing indexes are handled gracefully

>>> word[4:42]
'on'
>>> word[42:]
''

Python strings are immutable
need to create a new string to accommodate desired changes

>>> 'J' + word[1:]
'Jython'
>>> word[:2] + 'py'
'Pypy'

additional information

Text Sequence Type — str : Strings are examples of sequence types, and support the common operations supported by such types
String Methods : Strings support a large number of methods for basic transformations and searching
f-strings : String literals that have embedded expressions
Format String Syntax : Information about string formatting with str.format()
printf-style String Formatting : The old formatting operations invoked when strings are the left operand of the % operator are described in more detail here

Lists

list of comma-separated values between square brackets
usually items are all of the same type
lists can be indexed and sliced
support operations like concatenation
are mutable
use list.append to add new items to end of list
simple assignments in Python never copies data - pointers
slice operations return a new list - shallow copy
lists can contain lists

First Steps Towards Programming

>>> # Fibonacci series:
>>> # the sum of two elements defines the next
>>> a, b = 0, 1
>>> while a < 10:
...   print(a)
...   a, b = b, a+b
...
0
1
1
2
3
5
8

first line shows a multiple assignment
body of the loop is indented
indentation is Python's way of grouping statements
each line within a basic block must be indented by the same amount
the print() function writes the value of the argument(s) it is given
strings are printed without quotes and a space is inserted between items

>>> i = 256*256
>>> print('The value of i is', i)
The value of i is 65536

the keyword argument end can be used to avoid the newline after the output

>>> a, b = 0, 1
>>> while a < 1000:
...   print(a, end=',')
...   a, b = b, a+b
...
0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,