Python , Input and Output

1. Python
Input and Output
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. Been using print to see what programs are doing
Python Input and Output

3. Been using print to see what programs are doing
How to save data to files?
Python Input and Output

4. Been using print to see what programs are doing
How to save data to files?
And read ddaattaa ffrroomm tthheemm??
Python Input and Output

5. Been using print to see what programs are doing
How to save data to files?
And read ddaattaa ffrroomm tthheemm??
Python's solution looks very much like C's
Python Input and Output

6. Been using print to see what programs are doing
How to save data to files?
And read ddaattaa ffrroomm tthheemm??
Python's solution looks very much like C's
– A file is a sequence of bytes
Python Input and Output

7. Been using print to see what programs are doing
How to save data to files?
And read ddaattaa ffrroomm tthheemm??
Python's solution looks very much like C's
– A file is a sequence of bytes
– But it's often more useful to treat it as a sequence
of lines
Python Input and Output

8. Sample data file
Three things are certain:
Death, taxes, and lost data.
Guess wwhhiicchh hhaass ooccccuurrrreedd..
Errors have occurred.
We won't tell you where or why.
Lazy programmers.
With searching comes loss
and the presence of absence:
"My Thesis" not found.
A crash reduces
your expensive computer
to a simple stone.
Python Input and Output

9. How many characters in a file?
Python Input and Output

10. How many characters in a file?
bytes
Python Input and Output

11. How many characters in a file?
bytes Assume 1-to-1 for now
Python Input and Output

12. How many characters in a file?
bytes Assume 1-to-1 for now
RReevviissiitt llaatteerr
Python Input and Output

13. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
Python Input and Output

14. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
Create a file object
Python Input and Output

15. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
File to connect to
Python Input and Output

16. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
To read
Python Input and Output

17. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
Now holds file object
Python Input and Output

18. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
Read entire content
of file into a string
Python Input and Output

19. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
Now has a copy of
all the bytes that were
in the file
Python Input and Output

20. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
Disconnect from the file
Python Input and Output

21. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
Disconnect from the file
Not strictly necessary
in small programs, but
good practice
Python Input and Output

22. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
Report how many
characters were read
Python Input and Output

23. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
Report how many
characters were read
bytes
Python Input and Output

24. How many characters in a file?
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
print len(data)
293
Python Input and Output

25. If the file might be large, better to read in chunks
Python Input and Output

26. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
wwwwhhhhiiiilllleeee data != '':
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
Python Input and Output

27. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
Read (at wwwwhhhhiiiilllleeee data != '': most) 64 bytes
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
Python Input and Output

28. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
Read (at wwwwhhhhiiiilllleeee data != '': most) 64 bytes
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
Or the empty string
if there is no more data
Python Input and Output

29. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
wwwwhhhhiiiilllleeee data != '':
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
Keep looping as long as
the last read returned
some data
Python Input and Output

30. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
wwwwhhhhiiiilllleeee data != '':
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
Do something with
the data
Python Input and Output

31. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
wwwwhhhhiiiilllleeee data != '':
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
(Try to) reload
Python Input and Output

32. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
wwwwhhhhiiiilllleeee data != '':
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
Should be 0 (or the loop
would still be running)
Python Input and Output

33. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
wwwwhhhhiiiilllleeee data != '':
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
64
64
64
64
37
0
Python Input and Output

34. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
wwwwhhhhiiiilllleeee data != '':
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
64
64
Don't do this unless
64
64
37
0
Python Input and Output

35. If the file might be large, better to read in chunks
reader = open('haiku.txt', 'r')
data = reader.read(64)
Don't do this unless the file really
might be very large (or infinite)
wwwwhhhhiiiilllleeee data != '':
pppprrrriiiinnnntttt len(data)
data = reader.read(64)
pppprrrriiiinnnntttt len(data)
reader.close()
64
64
64 64
37
0
Python Input and Output

36. More common to read one line at a time
Python Input and Output

37. More common to read one line at a time
reader = open('haiku.txt', 'r')
line = reader.readline()
total = 0
count = 0
wwwwhhhhiiiilllleeee line != '':
count += 1
total += len(line)
line = reader.readline()
reader.close()
pppprrrriiiinnnntttt 'average', float(total) / float(count)
Python Input and Output

38. More common to read one line at a time
reader = open('haiku.txt', 'r')
line = reader.readline() Read a single line
total = 0
count = 0
wwwwhhhhiiiilllleeee line != '':
count += 1
total += len(line)
line = reader.readline()
reader.close()
pppprrrriiiinnnntttt 'average', float(total) / float(count)
Python Input and Output

39. More common to read one line at a time
reader = open('haiku.txt', 'r')
line = reader.readline()
total = 0
count = 0
wwwwhhhhiiiilllleeee line != '':
count += 1
total += len(line)
line = reader.readline()
reader.close()
Keep looping until
no more lines in file
pppprrrriiiinnnntttt 'average', float(total) / float(count)
Python Input and Output

40. More common to read one line at a time
reader = open('haiku.txt', 'r')
line = reader.readline()
total = 0
count = 0
wwwwhhhhiiiilllleeee line != '':
count += 1
total += len(line)
line = reader.readline()
reader.close()
(Try to) reload
pppprrrriiiinnnntttt 'average', float(total) / float(count)
Python Input and Output

41. More common to read one line at a time
reader = open('haiku.txt', 'r')
line = reader.readline()
total = 0
count = 0
wwwwhhhhiiiilllleeee line != '':
count += 1
total += len(line)
line = reader.readline()
reader.close()
pppprrrriiiinnnntttt 'average', float(total) / float(count)
19.53333333
Python Input and Output

42. Often more convenient to read all lines at once
Python Input and Output

43. Often more convenient to read all lines at once
reader = open('haiku.txt', 'r')
contents = reader.readlines()
reader.close()
total = 0
count = 0
ffffoooorrrr line iiiinnnn contents:
count += 1
total += len(line)
pppprrrriiiinnnntttt 'average', float(total) / float(count)
Python Input and Output

44. Often more convenient to read all lines at once
reader = open('haiku.txt', 'r')
contents = reader.readlines() All lines in file
reader.close()
total = 0
count = 0
ffffoooorrrr line iiiinnnn contents:
count += 1
total += len(line)
as list of strings
pppprrrriiiinnnntttt 'average', float(total) / float(count)
Python Input and Output

45. Often more convenient to read all lines at once
reader = open('haiku.txt', 'r')
contents = reader.readlines() Loop over lines
reader.close()
total = 0
count = 0
ffffoooorrrr line iiiinnnn contents:
count += 1
total += len(line)
with for
pppprrrriiiinnnntttt 'average', float(total) / float(count)
Python Input and Output

46. Often more convenient to read all lines at once
reader = open('haiku.txt', 'r')
contents = reader.readlines()
reader.close()
total = 0
count = 0
ffffoooorrrr line iiiinnnn contents:
count += 1
total += len(line)
pppprrrriiiinnnntttt 'average', float(total) / float(count)
19.53333333
Python Input and Output

47. "Read lines as list" + "loop over list" is common idiom
Python Input and Output

48. "Read lines as list" + "loop over list" is common idiom
So Python provides "loop over lines in file"
Python Input and Output

49. "Read lines as list" + "loop over list" is common idiom
So Python provides "loop over lines in file"
reader = open('haiku.txt', 'r')
total = 0
count = 0
ffffoooorrrr line iiiinnnn reader:
count += 1
total += len(line)
reader.close()
pppprrrriiiinnnntttt 'average', float(total) / float(count)
Python Input and Output

50. "Read lines as list" + "loop over list" is common idiom
So Python provides "loop over lines in file"
reader = open('haiku.txt', 'r')
total = 0
count = 0
ffffoooorrrr line iiiinnnn reader:
count += 1
total += len(line)
Assign lines of text in file
to loop variable one by one
reader.close()
pppprrrriiiinnnntttt 'average', float(total) / float(count)
Python Input and Output

51. "Read lines as list" + "loop over list" is common idiom
So Python provides "loop over lines in file"
reader = open('haiku.txt', 'r')
total = 0
count = 0
ffffoooorrrr line iiiinnnn reader:
count += 1
total += len(line)
reader.close()
pppprrrriiiinnnntttt 'average', float(total) / float(count)
19.53333333
Python Input and Output

52. Python Input and Output

53. Put data in a file using write or writelines
Python Input and Output

54. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elements')
writer.writelines(['He', 'Ne', 'Ar', 'Kr'])
writer.close()
Python Input and Output

55. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elements')
writer.writelines(['He', 'Ne', 'Ar', 'Kr'])
writer.close()
Same function
Python Input and Output

56. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elements')
writer.writelines(['He', 'Ne', 'Ar', 'Kr'])
writer.close()
File to write to
Python Input and Output

57. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elements')
writer.writelines(['He', 'Ne', 'Ar', 'Kr'])
writer.close()
File to write to
Created if it doesn't exist
Python Input and Output

58. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elements')
writer.writelines(['He', 'Ne', 'Ar', 'Kr'])
writer.close()
For writing instead
of reading
Python Input and Output

59. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elements')
writer.writelines(['He', 'Ne', 'Ar', 'Kr'])
writer.close()
Write a single string
Python Input and Output

60. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elements')
writer.writelines(['He', 'Ne', 'Ar', 'Kr'])
writer.close()
Write each string
in a list
Python Input and Output

61. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elements')
writer.writelines(['He', 'Ne', 'Ar', 'Kr'])
writer.close()
elementsHeNeArKr
Python Input and Output

62. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elements')
writer.writelines(['He', 'Ne', 'Ar', 'Kr'])
writer.close()
elementsHeNeArKr
Python only writes what you tell it to
Python Input and Output

63. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elementsn')
writer.writelines(['Hen', 'Nen', 'Arn', 'Krn'])
writer.close()
Have to provide end-of-line characters yourself
Python Input and Output

64. Put data in a file using write or writelines
writer = open('temp.txt', 'w')
writer.write('elementsn')
writer.writelines(['Hen', 'Nen', 'Arn', 'Krn'])
writer.close()
elements
He
Ne
Ar
Kr
Python Input and Output

65. Often simpler to use print >>
Python Input and Output

66. Often simpler to use print >>
writer = open('temp.txt', 'w')
pppprrrriiiinnnntttt >> writer, 'elements'
ffffoooorrrr gas iiiinnnn ['He', 'Ne', 'Ar', 'Kr']:
pppprrrriiiinnnntttt >> writer, gas
writer.close()
Python Input and Output

67. Often simpler to use print >>
writer = open('temp.txt', 'w')
pppprrrriiiinnnntttt >> writer, 'elements'
ffffoooorrrr gas iiiinnnn ['He', 'Ne', 'Ar', 'Kr']:
pppprrrriiiinnnntttt >> writer, gas
writer.close()
Specify open file after >>
Python Input and Output

68. Often simpler to use print >>
writer = open('temp.txt', 'w')
pppprrrriiiinnnntttt >> writer, 'elements'
ffffoooorrrr gas iiiinnnn ['He', 'Ne', 'Ar', 'Kr']:
pppprrrriiiinnnntttt >> writer, gas
writer.close()
print automatically adds the newline
Python Input and Output

69. Copy a file
Python Input and Output

70. Copy a file
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
writer = open('temp.txt', 'w')
write.write(data)
writer.close()
Python Input and Output

71. Copy a file
reader = open('haiku.txt', 'r')
data = reader.read() Read all
reader.close()
writer = open('temp.txt', 'w')
write.write(data)
writer.close()
Python Input and Output

72. Copy a file
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
writer = open('temp.txt', 'w')
write.write(data)
writer.close()
Write all
Python Input and Output

73. Copy a file
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
writer = open('temp.txt', 'w')
write.write(data)
writer.close()
Probably won't work with a terabyte...
Python Input and Output

74. Copy a file
reader = open('haiku.txt', 'r')
data = reader.read()
reader.close()
writer = open('temp.txt', 'w')
write.write(data)
writer.close()
Probably won't work with a terabyte...
…but we probably don't care
Python Input and Output

75. Copy a file (version 2)
Python Input and Output

76. Copy a file (version 2)
reader = open('haiku.txt', 'r')
writer = open('temp.txt', 'w')
ffffoooorrrr line iiiinnnn reader:
writer.write(line)
reader.close()
writer.close()
Python Input and Output

77. Copy a file (version 2)
reader = open('haiku.txt', 'r')
writer = open('temp.txt', 'w')
ffffoooorrrr line iiiinnnn reader:
writer.write(line)
reader.close()
writer.close()
Assumes the file is text
Python Input and Output

78. Copy a file (version 2)
reader = open('haiku.txt', 'r')
writer = open('temp.txt', 'w')
ffffoooorrrr line iiiinnnn reader:
writer.write(line)
reader.close()
writer.close()
Assumes the file is text
Or at least that the end-of-line character appears
frequently
Python Input and Output

79. This version doesn't make an exact copy
Python Input and Output

80. This version doesn't make an exact copy
reader = open('haiku.txt', 'r')
writer = open('temp.txt', 'w')
ffffoooorrrr line iiiinnnn reader:
print >> writer, line
reader.close()
writer.close()
Python Input and Output

81. This version doesn't make an exact copy
reader = open('haiku.txt', 'r')
writer = open('temp.txt', 'w')
ffffoooorrrr line iiiinnnn reader:
print >> writer, line
reader.close()
writer.close()
Python keeps the newline when reading
Python Input and Output

82. This version doesn't make an exact copy
reader = open('haiku.txt', 'r')
writer = open('temp.txt', 'w')
ffffoooorrrr line iiiinnnn reader:
print >> writer, line
reader.close()
writer.close()
Python keeps the newline when reading
print automatically adds a newline
Python Input and Output

83. This version doesn't make an exact copy
reader = open('haiku.txt', 'r')
writer = open('temp.txt', 'w')
ffffoooorrrr line iiiinnnn reader:
print >> writer, line
reader.close()
writer.close()
Python keeps the newline when reading
print automatically adds a newline
Result is double-spaced output
Python Input and Output

84. Copy a file (version 3)
Python Input and Output

85. Copy a file (version 3)
BLOCKSIZE = 1024
reader = open('haiku.txt', 'r')
writer = open('temp.txt', 'w')
data = reader.read(BLOCKSIZE)
wwwwhhhhiiiilllleeee len(data) > 0:
writer.write(data)
data = reader.read(BLOCKSIZE)
reader.close()
writer.close()
Python Input and Output

86. Copy a file (version 3)
BLOCKSIZE = 1024
reader = open('haiku.txt', 'r')
writer = open('temp.txt', 'w')
data = reader.read(BLOCKSIZE)
wwwwhhhhiiiilllleeee len(data) > 0:
writer.write(data)
data = reader.read(BLOCKSIZE)
reader.close()
writer.close()
(Needlessly?) harder to understand
Python Input and Output

87. created by
Greg Wilson
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.