Basics Copyright © Software Carpentry 2010 This work is licensed under the Creative Commons Attribution License See for more information. Python

Basics A simple interpreted language

PythonBasics A simple interpreted language no separate compilation step

PythonBasics $ python >>> A simple interpreted language no separate compilation step

PythonBasics $ python >>> print >>> A simple interpreted language no separate compilation step

PythonBasics $ python >>> print >>> print 'charles' + 'darwin' charlesdarwin >>> A simple interpreted language no separate compilation step

PythonBasics Put commands in a file and execute that

PythonBasics Put commands in a file and execute that $ nano very-simple.py

PythonBasics Put commands in a file and execute that $ nano very-simple.py print print 'charles' + 'darwin'

PythonBasics Put commands in a file and execute that $ nano very-simple.py print print 'charles' + 'darwin' $ python very-simple.py 3 charlesdarwin $

PythonBasics Use an integrated development environment (IDE)

PythonBasics Use an integrated development environment (IDE) Source file

PythonBasics Use an integrated development environment (IDE) Source file Execution shell

PythonBasics Variables are names for values

PythonBasics Variables are names for values Created by use

PythonBasics Variables are names for values Created by use: no declaration necessary

PythonBasics Variables are names for values Created by use: no declaration necessary >>> planet = 'Pluto' >>>

PythonBasics Variables are names for values Created by use: no declaration necessary >>> planet = 'Pluto' >>> print planet Pluto >>>

PythonBasics Variables are names for values Created by use: no declaration necessary variablevalue planet 'Pluto' >>> planet = 'Pluto' >>> print planet Pluto >>>

PythonBasics Variables are names for values Created by use: no declaration necessary >>> planet = 'Pluto' >>> print planet Pluto >>> moon = 'Charon' >>> variablevalue planet moon 'Pluto' 'Charon'

PythonBasics Variables are names for values Created by use: no declaration necessary >>> planet = 'Pluto' >>> print planet Pluto >>> moon = 'Charon' >>> p = planet >>> variablevalue planet moon 'Pluto' 'Charon'

PythonBasics Variables are names for values Created by use: no declaration necessary >>> planet = 'Pluto' >>> print planet Pluto >>> moon = 'Charon' >>> p = planet >>> variablevalue planet moon p 'Pluto' 'Charon'

PythonBasics Variables are names for values Created by use: no declaration necessary >>> planet = 'Pluto' >>> print planet Pluto >>> moon = 'Charon' >>> p = planet >>> print p Pluto >>> variablevalue planet moon p 'Pluto' 'Charon'

PythonBasics A variable is just a name

PythonBasics A variable is just a name Does not have a type

PythonBasics A variable is just a name Does not have a type >>> planet = 'Pluto' >>>

PythonBasics A variable is just a name Does not have a type >>> planet = 'Pluto' >>> planet 'Pluto' variablevalue string

PythonBasics A variable is just a name Does not have a type >>> planet = 'Pluto' >>> planet = 9 >>> planet 'Pluto' 9 variablevalue integer

PythonBasics A variable is just a name Does not have a type >>> planet = 'Pluto' >>> planet = 9 >>> planet 'Pluto' 9 variablevalue Values are garbage collected

PythonBasics A variable is just a name Does not have a type >>> planet = 'Pluto' >>> planet = 9 >>> planet 'Pluto' 9 variablevalue Values are garbage collected If nothing refers to data any longer, it can be recycled

PythonBasics A variable is just a name Does not have a type >>> planet = 'Pluto' >>> planet = 9 >>> planet 'Pluto' 9 variablevalue Values are garbage collected If nothing refers to data any longer, it can be recycled

PythonBasics Must assign value to variable before using it

PythonBasics Must assign value to variable before using it >>> planet = 'Sedna' >>>

PythonBasics Must assign value to variable before using it >>> planet = 'Sedna' >>> print plant # note the deliberate misspelling

PythonBasics Must assign value to variable before using it >>> planet = 'Sedna' >>> print plant # note the deliberate misspelling Traceback (most recent call last): print plant NameError: name 'plant' is not defined >>>

PythonBasics Must assign value to variable before using it Python does not assume default values for variables >>> planet = 'Sedna' >>> print plant # note the deliberate misspelling Traceback (most recent call last): print plant NameError: name 'plant' is not defined >>>

PythonBasics Must assign value to variable before using it Python does not assume default values for variables Doing so can mask many errors >>> planet = 'Sedna' >>> print plant # note the deliberate misspelling Traceback (most recent call last): print plant NameError: name 'plant' is not defined >>>

PythonBasics Must assign value to variable before using it Python does not assume default values for variables Doing so can mask many errors Anything from # to the end of the line is a comment >>> planet = 'Sedna' >>> print plant # note the deliberate misspelling Traceback (most recent call last): print plant NameError: name 'plant' is not defined >>>

PythonBasics Values do have types

PythonBasics Values do have types >>> string = "two" >>> number = 3 >>> print string * number # repeated concatenation twotwotwo >>>

PythonBasics Values do have types >>> string = "two" >>> number = 3 >>> print string * number # repeated concatenation twotwotwo >>> print string + number Traceback (most recent call last) number + string TypeError: cannot concatenate 'str' and 'int' objects >>>

PythonBasics Values do have types >>> string = "two" >>> number = 3 >>> print string * number # repeated concatenation twotwotwo >>> print string + number Traceback (most recent call last) number + string TypeError: cannot concatenate 'str' and 'int' objects >>> Would probably be safe here to produce 'two3'

PythonBasics Values do have types >>> string = "two" >>> number = 3 >>> print string * number # repeated concatenation twotwotwo >>> print string + number Traceback (most recent call last) number + string TypeError: cannot concatenate 'str' and 'int' objects >>> Would probably be safe here to produce 'two3' But then what should '2'+'3' be?

PythonBasics Values do have types >>> string = "two" >>> number = 3 >>> print string * number # repeated concatenation twotwotwo >>> print string + number Traceback (most recent call last) number + string TypeError: cannot concatenate 'str' and 'int' objects >>> Would probably be safe here to produce 'two3' But then what should '2'+'3' be? Doing too much is as bad as doing too little…

PythonBasics Use functions to convert between types

PythonBasics Use functions to convert between types >>> print int('2') >>>

PythonBasics Use functions to convert between types >>> print int('2') >>> print 2 + str(3) 23 >>>

PythonBasics Numbers

PythonBasics Numbers bit integer (on most machines)

PythonBasics Numbers bit integer (on most machines) bit float (ditto)

PythonBasics Numbers bit integer (on most machines) bit float (ditto) 1+4j complex number (two 64-bit floats)

PythonBasics Numbers bit integer (on most machines) bit float (ditto) 1+4j complex number (two 64-bit floats) x.real, x.imagreal and imaginary parts of complex number

PythonBasics Arithmetic

PythonBasics Arithmetic Addition

PythonBasics Arithmetic Addition 'Py' + 'thon''Python'

PythonBasics Arithmetic Addition 'Py' + 'thon''Python' Subtraction

PythonBasics Arithmetic Addition 'Py' + 'thon''Python' Subtraction Multiplication*3 * 26

PythonBasics Arithmetic Addition 'Py' + 'thon''Python' Subtraction Multiplication*3 * 26 'Py' * 2'PyPy'

PythonBasics Arithmetic Addition 'Py' + 'thon''Python' Subtraction Multiplication*3 * 26 'Py' * 2'PyPy' Division/3.0 / 21.5

PythonBasics Arithmetic Addition 'Py' + 'thon''Python' Subtraction Multiplication*3 * 26 'Py' * 2'PyPy' Division/3.0 / / 21

PythonBasics Arithmetic Addition 'Py' + 'thon''Python' Subtraction Multiplication*3 * 26 'Py' * 2'PyPy' Division/3.0 / / 21 Exponentiation**2 **

PythonBasics Arithmetic Addition 'Py' + 'thon''Python' Subtraction Multiplication*3 * 26 'Py' * 2'PyPy' Division/3.0 / / 21 Exponentiation**2 ** Remainder%13 % 53

PythonBasics Prefer in-place forms of binary operators

PythonBasics Prefer in-place forms of binary operators >>> years = 500 >>>

PythonBasics Prefer in-place forms of binary operators >>> years = 500 >>> years += 1 >>>

PythonBasics Prefer in-place forms of binary operators >>> years = 500 >>> years += 1 >>> The same as years = years + 1

PythonBasics Prefer in-place forms of binary operators >>> years = 500 >>> years += 1 >>> print years 501 >>>

PythonBasics Prefer in-place forms of binary operators >>> years = 500 >>> years += 1 >>> print years 501 >>> years %= 10 >>>

PythonBasics Prefer in-place forms of binary operators >>> years = 500 >>> years += 1 >>> print years 501 >>> years %= 10 >>> The same as years = years % 10

PythonBasics Prefer in-place forms of binary operators >>> years = 500 >>> years += 1 >>> print years 501 >>> years %= 10 >>> print years 5 >>>

PythonBasics Comparisons

PythonBasics Comparisons 3 < 5True

PythonBasics Comparisons 3 < 5True 3 != 5True

PythonBasics Comparisons 3 < 5True 3 != 5True 3 == 5False

PythonBasics Comparisons 3 < 5True 3 != 5True 3 == 5False Single = is assignment Double == is equality

PythonBasics Comparisons 3 < 5True 3 != 5True 3 == 5False 3 >= 5False

PythonBasics Comparisons 3 < 5True 3 != 5True 3 == 5False 3 >= 5False 1 < 3 < 5True

PythonBasics Comparisons 3 < 5True 3 != 5True 3 == 5False 3 >= 5False 1 < 3 < 5True 1 3True But please don't do this

PythonBasics Comparisons 3 < 5True 3 != 5True 3 == 5False 3 >= 5False 1 < 3 < 5True 1 3True 3+2j < 5error

October 2010 created by Greg Wilson Copyright © Software Carpentry 2010 This work is licensed under the Creative Commons Attribution License See for more information.