11.5 Alternative File Reading Methods

• In addition to the for loop, Python provides three methods to read data from the input file.
• The readline method reads one line from the file and returns it as a string.
  • The string returned by readline will contain the newline character at the end
  • This method returns the empty string when it reaches the end of the file.
  • readlines method returns the contents of the entire file as a list of strings
    • each item in the list represents one line of the file
• It is also possible to read the entire file into a single string with read
• Note that we need to reopen the file before each read so that we start from the beginning
• Each file has a marker that denotes the current read position in the file
  • Any time one of the read methods is called the marker is moved to the character immediately following the last character returned
  • readline this moves the marker to the first character of the next line in the file
  • read or readlines the marker is moved to the end of the file.

>>> infile = open("ccdata.txt", "r")
>>> aline = infile.readline()
>>> aline
'1850\-0.37\2.24E-7\n'
>>>
>>> infile = open("ccdata.txt", "r")
>>> linelist = infile.readlines()
>>> print(len(linelist))
18
>>> print(linelist[0:4])
['1850\-0.37\2.24E-7\n', 
'1860\-0.34\3.94E-7\n', 
'1870\-0.28\6.6E-7\n', 
'1880\-0.24\1.1\n']
>>>
>>> infile = open("ccdata.txt", "r")
>>> filestring = infile.read()
>>> print(len(filestring))
1282
>>> print(filestring[:256])
1850 -0.37 2.24E-7
1860 -0.34 3.94E-7
1870 -0.28 6.6E-7
1880 -0.24
>>>

Reading file with while loop

• This is important because many other programming languages do not support the for loop style for reading files but they do support the pattern we’ll show you here.

infile = open("ccdata.txt", "r")
line = infile.readline()
while line:
    values = line.split()
    print('In', values[0], 'the average temp. was', values[1], '°C and CO2 emmisions were', values[2], 'gigatons.')
    line = infile.readline()

infile.close()

In 1850 the average temp. was -0.37 °C and CO2 emmisions were 2.24E-7 gigatons.
In 1860 the average temp. was -0.34 °C and CO2 emmisions were 3.94E-7 gigatons.
In 1870 the average temp. was -0.28 °C and CO2 emmisions were 6.6E-7 gigatons.
In 1880 the average temp. was -0.24 °C and CO2 emmisions were 1.1 gigatons.
In 1890 the average temp. was -0.42 °C and CO2 emmisions were 1.72 gigatons.
In 1900 the average temp. was -0.2 °C and CO2 emmisions were 2.38 gigatons.
In 1910 the average temp. was -0.49 °C and CO2 emmisions were 3.34 gigatons.
In 1920 the average temp. was -0.25 °C and CO2 emmisions were 4.01 gigatons.
In 1930 the average temp. was -0.14 °C and CO2 emmisions were 4.53 gigatons.
In 1940 the average temp. was 0.01 °C and CO2 emmisions were 5.5 gigatons.
In 1950 the average temp. was -0.17 °C and CO2 emmisions were 6.63 gigatons.
In 1960 the average temp. was -0.05 °C and CO2 emmisions were 10.5 gigatons.
In 1970 the average temp. was -0.03 °C and CO2 emmisions were 16 gigatons.
In 1980 the average temp. was 0.09 °C and CO2 emmisions were 20.3 gigatons.
In 1990 the average temp. was 0.3 °C and CO2 emmisions were 22.6 gigatons.
In 2000 the average temp. was 0.29 °C and CO2 emmisions were 24.9 gigatons.
In 2010 the average temp. was 0.56 °C and CO2 emmisions were 32.7 gigatons.
In 2019 the average temp. was 0.74 °C and CO2 emmisions were 33.3 gigatons.

• There are several important things to notice in this code:
  • On line 2 we have the statement line = infile.readline()
    • We call this initial read the priming read.
    • It is very important because the while condition needs to have a value for the line variable.
  • readline method will return the empty string if there is no more data in the file.
    • When Python is looking for a Boolean condition, as in while line:, it treats an empty sequence type as False, and a non-empty sequence as True
    • So, the only way that a line of data from the file can be empty is if you are reading at the end of the file, and the while condition becomes False.
  • Notice that the last line of the body of the while loop performs another readline
    • This statement will reassign the variable line to the next line of the file
    • It represents the change of state that is necessary for the iteration to function correctly.
    • Without it, there would be an infinite loop processing the same line of data over and over.