holys holys - 4 months ago 22
Python Question

Confused by python file mode "w+"

From the doc,


Modes 'r+', 'w+' and 'a+' open the file for updating (note that 'w+' truncates the file). Append 'b' to the mode to open the file in binary mode, on systems that differentiate between binary and text files; on systems that don’t have this distinction, adding the 'b' has no effect.


and here


w+ : Opens a file for both writing and reading. Overwrites the existing file if the file exists. If the file does not exist, creates a new file for reading and writing.


But, how to read a file open with
w+
?

Answer

Let's say you're opening the file with a with statement like you should be. Then you'd do something like this to read from your file:

with open('somefile.txt', 'w+') as f:
    # Note that f has now been truncated to 0 bytes, so you'll only
    # be able to read data that you wrote earlier...
    f.write('somedata\n')
    f.seek(0)  # Important: return to the top of the file before reading, otherwise you'll just read an empty string
    data = f.read() # Returns 'somedata\n'

Note the f.seek(0) -- if you forget this, the f.read() call will try to read from the end of the file, and will return an empty string.