There are different modes you can open a file with, specified by the mode parameter. These include:

• 'r' - reading mode. The default. It allows you only to read the file, not to modify it. When using this mode the file must exist.
• 'w' - writing mode. It will create a new file if it does not exist, otherwise will erase the file and allow you to write to it.
• 'a' - append mode. It will write data to the end of the file. It does not erase the file, and the file must exist for this mode.
• 'rb' - reading mode in binary. This is similar to r except that the reading is forced in binary mode. This is also a default choice.
• 'r+' - reading mode plus writing mode at the same time. This allows you to read and write into files at the same time without having to use r and w.
• 'rb+' - reading and writing mode in binary. The same as r+ except the data is in binary
• 'wb' - writing mode in binary. The same as w except the data is in binary.
• 'w+' - writing and reading mode. The exact same as r+ but if the file does not exist, a new one is made. Otherwise, the file is overwritten.
• 'wb+' - writing and reading mode in binary mode. The same as w+ but the data is in binary.
• 'ab' - appending in binary mode. Similar to a except that the data is in binary.
• 'a+' - appending and reading mode. Similar to w+ as it will create a new file if the file does not exist. Otherwise, the file pointer is at the end of the file if it exists.
• 'ab+' - appending and reading mode in binary. The same as a+ except that the data is in binary.

with open(filename, 'r') as f:
    f.read()
with open(filename, 'w') as f:
    f.write(filedata)
with open(filename, 'a') as f:
    f.write('\\n' + newdata)

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Python 3 added a new mode for exclusive creation so that you will not accidentally truncate or overwrite and existing file.

• 'x' - open for exclusive creation, will raise FileExistsError if the file already exists
• 'xb' - open for exclusive creation writing mode in binary. The same as x except the data is in binary.
• 'x+' - reading and writing mode. Similar to w+ as it will create a new file if the file does not exist. Otherwise, will raise FileExistsError.
• 'xb+' - writing and reading mode. The exact same as x+ but the data is binary