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50 lines (38 loc) · 2.46 KB
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"""Methods of File Objects
@see: https://docs.python.org/3/tutorial/inputoutput.html#methods-of-file-objects
Reading from a file does not always have to be sequential. There are methods to look for
specific locations in the file, much like flipping to a page in a book.
"""
def test_file_methods():
"""Methods of File Objects"""
multi_line_file = open('src/files/multi_line_file.txt', 'r')
binary_file = open('src/files/binary_file', 'r')
# To read a file’s contents, call f.read(size), which reads some quantity of data and returns
# it as a string (in text mode) or bytes object (in binary mode). size is an optional numeric
# argument. When size is omitted or negative, the entire contents of the file will be read and
# returned; it’s your problem if the file is twice as large as your machine’s memory. Otherwise,
# at most size bytes are read and returned. If the end of the file has been reached, f.read()
# will return an empty string ('').
read_data = multi_line_file.read()
# pylint: disable=duplicate-code
assert read_data == 'first line\nsecond line\nthird line'
# To change the file object’s position, use f.seek(offset, from_what). The position is computed
# from adding offset to a reference point; the reference point is selected by the from_what
# argument. A from_what value of 0 measures from the beginning of the file, 1 uses the current
# file position, and 2 uses the end of the file as the reference point. from_what can be omitted
# and defaults to 0, using the beginning of the file as the reference point.
assert binary_file.seek(0) == 0 # Go to the 0th byte in the file
assert binary_file.seek(6) == 6 # Go to the 6th byte in the file
assert binary_file.read(1) == '6'
# f.readline() reads a single line from the file; a newline character (\n) is left at the end
# of the string, and is only omitted on the last line of the file if the file doesn’t end in a
# newline. This makes the return value unambiguous; if f.readline() returns an empty string,
# the end of the file has been reached, while a blank line is represented by '\n', a string
# containing only a single newline.
multi_line_file.seek(0)
assert multi_line_file.readline() == 'first line\n'
assert multi_line_file.readline() == 'second line\n'
assert multi_line_file.readline() == 'third line'
assert multi_line_file.readline() == ''
multi_line_file.close()
binary_file.close()