Open files in 'rt' and 'wt' modes

Question

Several times here on SO I've seen people using rt and wt modes for reading and writing files.

For example:

with open('input.txt', 'rt') as input_file:
     with open('output.txt', 'wt') as output_file: 
         ...

I don't see the modes documented, but since open() doesn't throw an error - looks like it's pretty much legal to use.

What is it for and is there any difference between using wt vs w and rt vs r?

Tammany

Answer 1

t refers to the text mode. There is no difference between r and rt or w and wt since text mode is the default.

Documented here:

Character   Meaning
'r'     open for reading (default)
'w'     open for writing, truncating the file first
'x'     open for exclusive creation, failing if the file already exists
'a'     open for writing, appending to the end of the file if it exists
'b'     binary mode
't'     text mode (default)
'+'     open a disk file for updating (reading and writing)
'U'     universal newlines mode (deprecated)

The default mode is 'r' (open for reading text, synonym of 'rt').

Thomsen

Gotcha, it's documented in python3 docs. So, there is basically no difference between wt vs w and rt vs r - just explicit is better than implicit? – Tammany

@Tammany Right, since text mode is the default, there is no difference between r and rt... – Thomsen

Note that w isn't always equal to wt. One such case is gzip.open where binary mode is default, and not text mode. Related question: #42013583 – Badderlocks

Answer 2

The t indicates text mode, meaning that \n characters will be translated to the host OS line endings when writing to a file, and back again when reading. The flag is basically just noise, since text mode is the default.

Other than U, those mode flags come directly from the standard C library's fopen() function, a fact that is documented in the sixth paragraph of the python2 documentation for open().

As far as I know, t is not and has never been part of the C standard, so although many implementations of the C library accept it anyway, there's no guarantee that they all will, and therefore no guarantee that it will work on every build of python. That explains why the python2 docs didn't list it, and why it generally worked anyway. The python3 docs make it official.

Eliathan

Answer 3

The 'r' is for reading, 'w' for writing and 'a' is for appending.

The 't' represents text mode as apposed to binary mode.

Several times here on SO I've seen people using rt and wt modes for reading and writing files.

Edit: Are you sure you saw rt and not rb?

These functions generally wrap the fopen function which is described here:

http://www.cplusplus.com/reference/cstdio/fopen/

As you can see it mentions the use of b to open the file in binary mode.

The document link you provided also makes reference to this b mode:

Appending 'b' is useful even on systems that don’t treat binary and text files differently, where it serves as documentation.

Haler

Yeah, it was clearly rt, e.g. #10971533, or #17128353 etc. Thank you for the info, good to know. – Tammany

In the link that devnull provides the 't' text option is listed. What surprised me was the C++ link did not also mention that 't' option as I'm pretty sure I'd used the 'rt' and 'wt' options in C fopen code written years ago. – Haler

Yeah, that's why I've asked - it was like a non-documented feature for me. Hope the thread would help someone in the future. Thanks again. – Tammany

Answer 4

t indicates for text mode

https://docs.python.org/release/3.1.5/library/functions.html#open

on linux, there's no difference between text mode and binary mode, however, in windows, they converts \n to \r\n when text mode.

http://www.cygwin.com/cygwin-ug-net/using-textbinary.html

Chirography

In Python 3 there's an additional difference between text and binary file modes (on all platforms). In text mode, read returns Unicode strings. In binary mode, read returns a bytes instance. If you want to write Python 2 code with forwards compatibility in mind, you can use io.open rather than the standard open to get the Python 3 behavior (with unicode versus str instances). – Breedlove