Python - Strings

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

2. Strings are sequences of characters
Python Strings

3. Strings are sequences of characters
No separate character type: just a string of length 1
Python Strings

4. Strings are sequences of characters
No separate character type: just a string of length 1
Indexed exactly like lists
Python Strings

5. Strings are sequences of characters
No separate character type: just a string of length 1
Indexed exactly like lists
name = 'Darwin'
print name[0], name[-1]
D n
Python Strings

6. for iterates through characters
Python Strings

7. for iterates through characters
name = 'Darwin'
for c in name:
print c
Darwin
Python Strings

8. Use either ' or " (as long as they match)
Python Strings

9. Use either ' or " (as long as they match)
print 'Alan', "Turing"
Alan Turing
Python Strings

10. Use either ' or " (as long as they match)
print 'Alan', "Turing"
Alan Turing
Strings are the same no matter how they're created
Python Strings

11. Use either ' or " (as long as they match)
print 'Alan', "Turing"
Alan Turing
Strings are the same no matter how they're created
print 'Alan' == "Alan"
True
Python Strings

12. Strings are compared character by character
from left to right
Python Strings

13. Strings are compared character by character
from left to right
print 'a' < 'b'
True
Python Strings

14. Strings are compared character by character
from left to right
print 'a' < 'b'
True
print 'ab' < 'abc'
True
Python Strings

15. Strings are compared character by character
from left to right
print 'a' < 'b'
True
print 'ab' < 'abc'
True
print '1' < '9'
True
Python Strings

16. Strings are compared character by character
from left to right
print 'a' < 'b'
True
print 'ab' < 'abc'
True
print '1' < '9'
True
print '100' < '9'
True
Python Strings

17. Strings are compared character by character
from left to right
print 'a' < 'b'
True
print 'ab' < 'abc'
True
print '1' < '9'
True
print '100' < '9'
True
print 'A' < 'a'
True
Python Strings

18. Strings are immutable : cannot be changed in place
Python Strings

19. Strings are immutable : cannot be changed in place
name = 'Darwin'
name[0] = 'C'
TypeError: 'str' object does not support item assignment
Python Strings

20. Strings are immutable : cannot be changed in place
name = 'Darwin'
name[0] = 'C'
TypeError: 'str' object does not support item assignment
Immutability improves performance
Python Strings

21. Strings are immutable : cannot be changed in place
name = 'Darwin'
name[0] = 'C'
TypeError: 'str' object does not support item assignment
Immutability improves performance
See later how immutability improves programmers'
performance
Python Strings

22. Use + to concatenate strings
Python Strings

23. Use + to concatenate strings
name = 'Charles' + ' ' + 'Darwin'
print name
Charles Darwin
Python Strings

24. Use + to concatenate strings
name = 'Charles' + ' ' + 'Darwin'
print name
Charles Darwin
Concatenation always produces a new string
Python Strings

25. Use + to concatenate strings
name = 'Charles' + ' ' + 'Darwin'
print name
Charles Darwin
Concatenation always produces a new string
original = 'Charles' original 'Charles'
Python Strings

26. Use + to concatenate strings
name = 'Charles' + ' ' + 'Darwin'
print name
Charles Darwin
Concatenation always produces a new string
'Charles'
original = 'Charles' original
name = original
name
Python Strings

27. Use + to concatenate strings
name = 'Charles' + ' ' + 'Darwin'
print name
Charles Darwin
Concatenation always produces a new string
original = 'Charles'
name = original
name += ' Darwin'
'Charles'
original
name 'Charles Darwin'
Python Strings

28. Often used to format output
Python Strings

29. Often used to format output
print 'reagant: ' + str(reagant_id) + ' produced ' + str(percentage_yield) + '% yield'
Python Strings

30. Often used to format output
print 'reagant: ' + str(reagant_id) + ' produced ' + str(percentage_yield) + '% yield'
There's a better way...
Python Strings

31. Use string % value to format output
Python Strings

32. Use string % value to format output
output = 'reagant: %d' % 123
print output
reagant: 123
Python Strings

33. Use string % value to format output
output = 'reagant: %d' % 123
print output
reagant: 123
percentage_yield = 12.3
print 'yield: %6.2f' % percentage_yield
yield: 12.30
Python Strings

34. And string % (v1, v2, ...) for multiple values
Python Strings

35. And string % (v1, v2, ...) for multiple values
reagant_id = 123
percentage_yield = 12.3
print 'reagant: %d produced %f%% yield' %
(reagant_id, percentage_yield)
reagant: 123 produced 12.30% yield
Python Strings

36. And string % (v1, v2, ...) for multiple values
reagant_id = 123
percentage_yield = 12.3
print 'reagant: %d produced %f%% yield' %
(reagant_id, percentage_yield)
reagant: 123 produced 12.30% yield
% operator turns double '%%' into single '%'
Python Strings

37. Use n to represent a newline character
Python Strings

38. Use n to represent a newline character
Use ' for single quote, " for double quote
Python Strings

39. Use n to represent a newline character
Use ' for single quote, " for double quote
print 'There isn't timento do it right.'
There isn't time
to do it right.
Python Strings

40. Use n to represent a newline character
Use ' for single quote, " for double quote
print 'There isn't timento do it right.'
There isn't time
to do it right.
print "But you said,n"There is time to do it over.""
But you said,
"There is time to do it over."
Python Strings

41. Use for a literal character
Python Strings

42. Use for a literal character
print 'Most mathematicians write ab instead of a%b.'
Most mathematicians write ab instead of a%b.
Python Strings

43. Use for a literal character
print 'Most mathematicians write ab instead of a%b.'
Most mathematicians write ab instead of a%b.
Common pattern with escape sequences
Python Strings

44. Use for a literal character
print 'Most mathematicians write ab instead of a%b.'
Most mathematicians write ab instead of a%b.
Common pattern with escape sequences
– Use a character to mean "what follows is special"
Python Strings

45. Use for a literal character
print 'Most mathematicians write ab instead of a%b.'
Most mathematicians write ab instead of a%b.
Common pattern with escape sequences
– Use a character to mean "what follows is special"
– Double it up to mean "that character itself"
Python Strings

46. Use triple quotes (either kind) for multi-line strings
Python Strings

47. Use triple quotes (either kind) for multi-line strings
quote = '''We can only see
a short distance ahead,
but we can see plenty there
that needs to be done.'''
Python Strings

48. Use triple quotes (either kind) for multi-line strings
quote = '''We can only see
a short distance ahead,
but we can see plenty there
that needs to be done.'''
d , n b u
Python Strings

49. Use triple quotes (either kind) for multi-line strings
quote = '''We can only see
a short distance ahead,
but we can see plenty there
that needs to be done.'''
quote = 'We can only seena short distance aheadn' 'but we can see plenty therenthat needs to be done.'
Python Strings

50. Strings have methods
Python Strings

51. Strings have methods
name = 'newTON'
print name.capitalize(), name.upper(), name.lower(), Newton NEWTON newton newTON
Python Strings

52. Strings have methods
name = 'newTON'
print name.capitalize(), name.upper(), name.lower(), Newton NEWTON newton newTON
dna = 'acggtggtcac'
print dna.count('g'), dna.count('x')
4 0
Python Strings

53. Strings have methods
name = 'newTON'
print name.capitalize(), name.upper(), name.lower(), Newton NEWTON newton newTON
dna = 'acggtggtcac'
print dna.count('g'), dna.count('x')
4 0
print dna.find('t'), dna.find('t', 5), dna.find('x')
4 7 -1
Python Strings

54. Strings have methods
name = 'newTON'
print name.capitalize(), name.upper(), name.lower(), Newton NEWTON newton newTON
dna = 'acggtggtcac'
print dna.count('g'), dna.count('x')
4 0
print dna.find('t'), dna.find('t', 5), dna.find('x')
4 7 -1
print dna.replace('t', 'x'), dna
acggxggxcac acggtggtcac
Python Strings

55. Strings have methods
name = 'newTON'
print name.capitalize(), name.upper(), name.lower(), Newton NEWTON newton newTON
dna = 'acggtggtcac'
print dna.count('g'), dna.count('x')
4 0
print dna.find('t'), dna.find('t', 5), dna.find('x')
4 7 -1
print dna.replace('t', 'x')
acggxggxcac acggtggtcac
print dna.replace('gt', '')
acggcac
Python Strings

56. Can chain method calls together
Python Strings

57. Can chain method calls together
element = 'cesium'
print element.upper().center(10, '.')
Python Strings

58. Can chain method calls together
element = 'cesium'
print element.upper().center(10, '.')
convert to upper case
Python Strings

59. Can chain method calls together
element = 'cesium'
print element.upper().center(10, '.')
center in a field
10 characters wide
Python Strings

60. Can chain method calls together
element = 'cesium'
print element.upper().center(10, '.')
..CESIUM..
Python Strings

61. narrated by
Dominique Vuvan
October 2010
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See http://software-carpentry.org/license.html for more information.