1. Basics Copyright © Software Carpentry 2010 This work is licensed under the Creative Commons Attribution License See http://software-carpentry.org/license.html for more information. Python

2. Basics A simple interpreted language

3. PythonBasics A simple interpreted language no separate compilation step

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

5. PythonBasics $ python >>> print 1 + 2 3 >>> A simple interpreted language no separate compilation step

6. PythonBasics $ python >>> print 1 + 2 3 >>> print 'charles' + 'darwin' charlesdarwin >>> A simple interpreted language no separate compilation step

7. PythonBasics Put commands in a file and execute that

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

9. PythonBasics Put commands in a file and execute that $ nano very-simple.py print 1 + 2 print 'charles' + 'darwin'

10. PythonBasics Put commands in a file and execute that $ nano very-simple.py print 1 + 2 print 'charles' + 'darwin' $ python very-simple.py 3 charlesdarwin $

11. PythonBasics Use an integrated development environment (IDE)

12. PythonBasics Use an integrated development environment (IDE) Source file

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

14. PythonBasics Variables are names for values

15. PythonBasics Variables are names for values Created by use

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

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

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

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

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

21. 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'

22. 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'

23. 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'

24. PythonBasics A variable is just a name

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

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

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

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

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

30. 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

31. 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

32. PythonBasics Must assign value to variable before using it

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

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

35. 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 >>>

36. 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 >>>

37. 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 >>>

38. 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 >>>

39. PythonBasics Values do have types

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

41. 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 >>>

42. 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'

43. 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?

44. 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…

45. PythonBasics Use functions to convert between types

46. PythonBasics Use functions to convert between types >>> print int('2') + 3 5 >>>

47. PythonBasics Use functions to convert between types >>> print int('2') + 3 5 >>> print 2 + str(3) 23 >>>

48. PythonBasics Numbers

49. PythonBasics Numbers 14 32-bit integer (on most machines)

50. PythonBasics Numbers 14 32-bit integer (on most machines) 14.0 64-bit float (ditto)

51. PythonBasics Numbers 14 32-bit integer (on most machines) 14.0 64-bit float (ditto) 1+4j complex number (two 64-bit floats)

52. PythonBasics Numbers 14 32-bit integer (on most machines) 14.0 64-bit float (ditto) 1+4j complex number (two 64-bit floats) x.real, x.imagreal and imaginary parts of complex number

53. PythonBasics Arithmetic

54. PythonBasics Arithmetic Addition+35 + 2257

55. PythonBasics Arithmetic Addition+35 + 2257 'Py' + 'thon''Python'

56. PythonBasics Arithmetic Addition+35 + 2257 'Py' + 'thon''Python' Subtraction-35 - 2213

57. PythonBasics Arithmetic Addition+35 + 2257 'Py' + 'thon''Python' Subtraction-35 - 2213 Multiplication*3 * 26

58. PythonBasics Arithmetic Addition+35 + 2257 'Py' + 'thon''Python' Subtraction-35 - 2213 Multiplication*3 * 26 'Py' * 2'PyPy'

59. PythonBasics Arithmetic Addition+35 + 2257 'Py' + 'thon''Python' Subtraction-35 - 2213 Multiplication*3 * 26 'Py' * 2'PyPy' Division/3.0 / 21.5

60. PythonBasics Arithmetic Addition+35 + 2257 'Py' + 'thon''Python' Subtraction-35 - 2213 Multiplication*3 * 26 'Py' * 2'PyPy' Division/3.0 / 21.5 3 / 21

61. PythonBasics Arithmetic Addition+35 + 2257 'Py' + 'thon''Python' Subtraction-35 - 2213 Multiplication*3 * 26 'Py' * 2'PyPy' Division/3.0 / 21.5 3 / 21 Exponentiation**2 ** 0.51.41421356...

62. PythonBasics Arithmetic Addition+35 + 2257 'Py' + 'thon''Python' Subtraction-35 - 2213 Multiplication*3 * 26 'Py' * 2'PyPy' Division/3.0 / 21.5 3 / 21 Exponentiation**2 ** 0.51.41421356... Remainder%13 % 53

63. PythonBasics Prefer in-place forms of binary operators

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

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

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

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

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

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

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

71. PythonBasics Comparisons

72. PythonBasics Comparisons 3 < 5True

73. PythonBasics Comparisons 3 < 5True 3 != 5True

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

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

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

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

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

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

80. October 2010 created by Greg Wilson Copyright © Software Carpentry 2010 This work is licensed under the Creative Commons Attribution License See http://software-carpentry.org/license.html for more information.