How to create and write to a textiowrapper and readlines

Question

So I am trying to create a text io wrapper that I can then use readlines() from for a unittest. Here is my attempt, but when I run it the readlines() returns nothing:

output = io.BytesIO()
wrapper =  io.TextIOWrapper(
 output,
 encoding='cp1252',
 line_buffering=True,
 )

wrapper.write('Text1')
wrapper.write('Text2')
wrapper.write('Text3')
wrapper.write('Text4')

for line in wrapper.readlines():
    print(line)

What do I need to change to get this output:

 Text1
 Text2
 Text3
 Text4

Yes the new answer worked. Looks like I was missing the `wrapper.seek(0,0)` call to start the stream. — EliSquared, Jun 29 '17 at 14:20

JosefZ · Accepted Answer · 2017-06-29T11:18:03.527

Read about TextIOWrapper class in the io module documentation:

A buffered text stream over a BufferedIOBase binary stream.

Edit: use seek function:

seek(offset[, whence])
Change the stream position to the given byte offset. offset is interpreted relative to the position indicated by whence. The default value for whence is SEEK_SET. Values for whence are:

SEEK_SET or 0 – start of the stream (the default); offset should be zero or positive

SEEK_CUR or 1 – current stream position; offset may be negative

SEEK_END or 2 – end of the stream; offset is usually negative

Return the new absolute position.

New in version 3.1: The SEEK_* constants.

New in version 3.3: Some operating systems could support additional values, like os.SEEK_HOLE or os.SEEK_DATA. The valid values for a file could depend on it being open in text or binary mode.

Try the following commented code snippet:

import io, os
output  = io.BytesIO()
wrapper = io.TextIOWrapper(
 output,
 encoding='cp1252',
 # errors=None,         #  defalut
 # newline=None,        #  defalut
 line_buffering=True,
 # write_through=False  #  defalut
 )

wrapper.write('Text1\n')
wrapper.write('Text2\n')
wrapper.write('Text3\n')
# wrapper.flush()                #  If line_buffering is True, flush() is implied
                                 ## when a call to write contains a newline character.

wrapper.seek(0,0)                #  start of the stream
for line in wrapper.readlines():
    print(line)

The rest of my original answer:

print(output.getvalue())         # for gebugging purposes

print( wrapper.write('Text4\n')) #  for gebugging purposes

# for line in wrapper.read():
for line in output.getvalue().decode('cp1252').split(os.linesep):
    print(line)

Output:

==> D:\test\Python\q44702487.py
b'Text1\r\nText2\r\nText3\r\n'
6
Text1
Text2
Text3
Text4

==>

The problem is that the object of `output.getvalue().decode('cp1252')` does not have have the property `readlines()` and if I do `wrapper.readlines()` nothing is returned. Is there any way to change the object so that you can use readlines()? I don't want to change the functionality of my code to get the unittest to work. — EliSquared, Jun 28 '17 at 13:19

How to create and write to a textiowrapper and readlines

1 Answers1