![]() I needed a function to go through all subdirectories of folder and edit content of the files based on some criteria, if it helps: new_file_content = ""įor directories, subdirectories, files in os.walk(folder_path):įile_path = os.path.join(directories, file_name) You can read, modify and save to the same file in python but you have actually to replace the whole content in file, and to call before updating file content: # set the pointer to the beginning of the file in order to rewrite the content You can find more information about file IO in the official Python docs. Or the most portable option is to use os.path.join(): fd = open(os.path.join("C:\\", "Users", "johndoe", "Desktop", "testfile.txt"), "r+") Or you can use raw strings by putting an r at the start: fd = open(r"C:\Users\johndoe\Desktop\testfile.txt", "r+") One final point - if you're specifying paths on Windows as literal strings, remember to escape your backslashes: fd = open("C:\\Users\\johndoe\\Desktop\\testfile.txt", "r+") If you're doing only reads or only writes then it's pretty safe. In summary, if you want to read/write from the middle of a file in Windows I'd suggest always using an explicit seek() instead of relying on the position of the read/write pointer. However, the second example (using seek()) seems to work fine on Windows. For example, when I execute that first example on my Windows machine then it correctly prints This, but when I check the file afterwards the write() has been completely ignored. However, I've had problems mixing read() and write() on Windows systems. Then you'll find that the file now contains This IS a TEST file.Īll this applies on Unix systems, and you can test those examples to make sure. After the example above, if you executed the following: fd = open("testfile.txt", "r+") You can use the seek() method of the file to jump to a specific point. It leaves the pointer at the space character just after This, so the following write(" IS") overwrites the next three characters with a space (the same as is already there) followed by IS, replacing the existing is. This is because the initial read(4) returns the first 4 characters of the file, because the pointer is at the start of the file. It should end up printing This and then leaving the file content as This IS a test file. So, if you executed the following code: fd = open("testfile.txt", "w+") ![]() I find it helps to think of this like an old cassette tape, if you remember those. Whenever you read or write N characters, the read/write pointer will move forward that amount within the file. delete all the contents) even if you add + to the mode. It's also worth noting that the w mode will truncate your file (i.e. Normally this defaults to the start of the file, but if you use a mode of a (append) then it defaults to the end of the file. That differentiate between binary and text files on systems thatĭon’t have this distinction, adding the 'b' has no effect.Įvery open file has an implicit pointer which indicates where data will be read and written. The string "foooooooooooooo" got appended at the end of the file since the pointer was already at the end of the file.Īre you on a system that differentiates between binary and text files? You might want to use 'rb+' as a mode in that case.Īppend 'b' to the mode to open the file in binary mode, on systems 'Test abc\nfasdfafasdfa\nsdfgsd\nfoooooooooooooo' In your case, it will append the string "Test abc" to the start of the file. In 'r+' mode, using write method will write the string object to the file based on where the pointer is. ![]() It is apparently not true if you are using Windows. My original response demonstrates how reading/writing on the same file opened for appending works. Intend could be obtained reliably by inserting You expect to happen, but seems most likely that what you ![]() The effect of mixing reads with writes on a file open for update isĮntirely undefined unless a file-positioning operation occurs between Quoting from the workaround explained at This seems like a bug specific to Windows. ![]()
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