simple test to ensure that optimizations in the io library deliver the expected results for best testing run this under a debugbuild python too to exercise asserts in the c code write s n s to file then open it and ensure that successive readlines deliver what we wrote ensure we can open testfn for writing since c doesn t guarantee we can writeread arbitrary bytes in text files use binary mode write once with n and once without repeat string pattern as often as needed to reach total length length then call tryone with that string a string one larger than that and a string one smaller than that try this with all small sizes and various powers of 2 so we exercise all likely stdio buffer sizes and off by one errors on both sides a pattern with prime length to avoid simple relationships with stdio buffer sizes c implementation python implementation simple test to ensure that optimizations in the io library deliver the expected results for best testing run this under a debug build python too to exercise asserts in the c code write s n s to file then open it and ensure that successive readline s deliver what we wrote ensure we can open testfn for writing since c doesn t guarantee we can write read arbitrary bytes in text files use binary mode write once with n and once without must be at eof repeat string pattern as often as needed to reach total length length then call try_one with that string a string one larger than that and a string one smaller than that try this with all small sizes and various powers of 2 so we exercise all likely stdio buffer sizes and off by one errors on both sides a pattern with prime length to avoid simple relationships with stdio buffer sizes
import unittest
from test.support import os_helper

import io 
import _pyio as pyio 





lengths = list(range(1, 257)) + [512, 1000, 1024, 2048, 4096, 8192, 10000,
                                 16384, 32768, 65536, 1000000]

class BufferSizeTest:
    def try_one(self, s):
        
        

        
        os_helper.unlink(os_helper.TESTFN)

        
        
        f = self.open(os_helper.TESTFN, "wb")
        try:
            
            f.write(s)
            f.write(b"\n")
            f.write(s)
            f.close()
            f = open(os_helper.TESTFN, "rb")
            line = f.readline()
            self.assertEqual(line, s + b"\n")
            line = f.readline()
            self.assertEqual(line, s)
            line = f.readline()
            self.assertFalse(line) 
            f.close()
        finally:
            os_helper.unlink(os_helper.TESTFN)

    def drive_one(self, pattern):
        for length in lengths:
            
            
            
            
            
            q, r = divmod(length, len(pattern))
            teststring = pattern * q + pattern[:r]
            self.assertEqual(len(teststring), length)
            self.try_one(teststring)
            self.try_one(teststring + b"x")
            self.try_one(teststring[:-1])

    def test_primepat(self):
        
        
        self.drive_one(b"1234567890\00\01\02\03\04\05\06")

    def test_nullpat(self):
        self.drive_one(b'\0' * 1000)


class CBufferSizeTest(BufferSizeTest, unittest.TestCase):
    open = io.open

class PyBufferSizeTest(BufferSizeTest, unittest.TestCase):
    open = staticmethod(pyio.open)


if __name__ == "__main__":
    unittest.main()