How do I append to a file?
Set the mode in open() to «a» (append) instead of «w» (write):
with open("test.txt", "a") as myfile: myfile.write("appended text") The documentation lists all the available modes.
bluewoodtree: The benefits are similar to that of RAII in C++. If you forget close(), it might take a while before the file is actually closed. It is easier that you might think to forget it when the code has multiple exit points, exceptions and so on.
There is a functional difference besides just remembering to close. with opens a context manager which will close the file even if there is an error between opening and close() .
One could easy do with open(«test.txt») as myfile: myfile.write(«appended text»,’a’) , but a is needed in open.
You need to open the file in append mode, by setting «a» or «ab» as the mode. See open().
When you open with «a» mode, the write position will always be at the end of the file (an append). You can open with «a+» to allow reading, seek backwards and read (but all writes will still be at the end of the file!).
>>> with open('test1','wb') as f: f.write('test') >>> with open('test1','ab') as f: f.write('koko') >>> with open('test1','rb') as f: f.read() 'testkoko' Note: Using ‘a’ is not the same as opening with ‘w’ and seeking to the end of the file — consider what might happen if another program opened the file and started writing between the seek and the write. On some operating systems, opening the file with ‘a’ guarantees that all your following writes will be appended atomically to the end of the file (even as the file grows by other writes).
A few more details about how the «a» mode operates (tested on Linux only). Even if you seek back, every write will append to the end of the file:
>>> f = open('test','a+') # Not using 'with' just to simplify the example REPL session >>> f.write('hi') >>> f.seek(0) >>> f.read() 'hi' >>> f.seek(0) >>> f.write('bye') # Will still append despite the seek(0)! >>> f.seek(0) >>> f.read() 'hibye' In fact, the fopen manpage states:
Opening a file in append mode (a as the first character of mode) causes all subsequent write operations to this stream to occur at end-of-file, as if preceded the call:
Old simplified answer (not using with ):
Example: (in a real program use with to close the file — see the documentation)
>>> open("test","wb").write("test") >>> open("test","a+b").write("koko") >>> open("test","rb").read() 'testkoko' So this implies, that multiple handles can be held across multiple processes, without any write conflicts?
f = open('filename.txt', 'a') f.write("stuff") f.close() It’s simple, but very useful.
Python has many variations off of the main three modes, these three modes are:
'w' write text 'r' read text 'a' append text So to append to a file it’s as easy as:
f = open('filename.txt', 'a') f.write('whatever you want to write here (in append mode) here.') Then there are the modes that just make your code fewer lines:
'r+' read + write text 'w+' read + write text 'a+' append + read text Finally, there are the modes of reading/writing in binary format:
'rb' read binary 'wb' write binary 'ab' append binary 'rb+' read + write binary 'wb+' read + write binary 'ab+' append + read binary You probably want to pass «a» as the mode argument. See the docs for open().
with open("foo", "a") as f: f.write("cool beans. ") There are other permutations of the mode argument for updating (+), truncating (w) and binary (b) mode but starting with just «a» is your best bet.
@MarkTolonen: file is no longer a builtin in Python 3. Even in Python 2, it is used very rarely. Opening a file is a common operation. It is ok to use file name here on both Python 2 and 3. Know when to be inconsistent.
You can also do it with print instead of write :
with open('test.txt', 'a') as f: print('appended text', file=f) If test.txt doesn’t exist, it will be created.
when we using this line open(filename, «a») , that a indicates the appending the file, that means allow to insert extra data to the existing file.
You can just use this following lines to append the text in your file
def FileSave(filename,content): with open(filename, "a") as myfile: myfile.write(content) FileSave("test.txt","test1 \n") FileSave("test.txt","test2 \n") The ‘a’ parameter signifies append mode. If you don’t want to use with open each time, you can easily write a function to do it for you:
def append(txt='\nFunction Successfully Executed', file): with open(file, 'a') as f: f.write(txt) If you want to write somewhere else other than the end, you can use ‘r+’ † :
import os with open(file, 'r+') as f: f.seek(0, os.SEEK_END) f.write("text to add") Finally, the ‘w+’ parameter grants even more freedom. Specifically, it allows you to create the file if it doesn’t exist, as well as empty the contents of a file that currently exists.
You can also open the file in r+ mode and then set the file position to the end of the file.
import os with open('text.txt', 'r+') as f: f.seek(0, os.SEEK_END) f.write("text to add") Opening the file in r+ mode will let you write to other file positions besides the end, while a and a+ force writing to the end.
if you want to append to a file
with open("test.txt", "a") as myfile: myfile.write("append me") We declared the variable myfile to open a file named test.txt . Open takes 2 arguments, the file that we want to open and a string that represents the kinds of permission or operation we want to do on the file
here is file mode options
Mode Description 'r' This is the default mode. It Opens file for reading. 'w' This Mode Opens file for writing. If file does not exist, it creates a new file. If file exists it truncates the file. 'x' Creates a new file. If file already exists, the operation fails. 'a' Open file in append mode. If file does not exist, it creates a new file. 't' This is the default mode. It opens in text mode. 'b' This opens in binary mode. '+' This will open a file for reading and writing (updating)
If multiple processes are writing to the file, you must use append mode or the data will be scrambled. Append mode will make the operating system put every write, at the end of the file irrespective of where the writer thinks his position in the file is. This is a common issue for multi-process services like nginx or apache where multiple instances of the same process, are writing to the same log file. Consider what happens if you try to seek, then write:
Example does not work well with multiple processes: f = open("logfile", "w"); f.seek(0, os.SEEK_END); f.write("data to write"); writer1: seek to end of file. position 1000 (for example) writer2: seek to end of file. position 1000 writer2: write data at position 1000 end of file is now 1000 + length of data. writer1: write data at position 1000 writer1's data overwrites writer2's data. By using append mode, the operating system will place any write at the end of the file.
f = open("logfile", "a"); f.seek(0, os.SEEK_END); f.write("data to write"); Append most does not mean, «open file, go to end of the file once after opening it». It means, «open file, every write I do will be at the end of the file».
WARNING: For this to work you must write all your record in one shot, in one write call. If you split the data between multiple writes, other writers can and will get their writes in between yours and mangle your data.
Python append to a file
While reading or writing to a file, access mode governs the type of operations possible in the opened file. It refers to how the file will be used once it’s opened. These modes also define the location of the File Handle in the file. The definition of these access modes is as follows:
- Append Only (‘a’): Open the file for writing.
- Append and Read (‘a+’): Open the file for reading and writing.
When the file is opened in append mode in Python, the handle is positioned at the end of the file. The data being written will be inserted at the end, after the existing data.
Example 1: Python program to illustrate Append vs write mode.
Python3
Output of Readlines after appending This is Delhi This is Paris This is LondonToday Output of Readlines after writing Tomorrow
Example 2: Append data from a new line
In the above example of file handling, it can be seen that the data is not appended from the new line. This can be done by writing the newline ‘\n’ character to the file.
Python3
Output of Readlines after appending This is Delhi This is Paris This is London TodayTomorrow
Note: ‘\n’ is treated as a special character of two bytes.
Example 3: Using With statement in Python
with statement is used in exception handling to make the code cleaner and much more readable. It simplifies the management of common resources like file streams. Unlike the above implementations, there is no need to call file.close() when using with statement. The with statement itself ensures proper acquisition and release of resources.
Python3
Hello This is Delhi This is Paris This is London Today
Note: To know more about with statement click here.
Using the shutil module:
This approach uses the shutil.copyfileobj() method to append the contents of another file (source_file) to ‘file.txt’. This can be useful if you want to append the contents of one file to another without having to read the contents into memory first.
Approach:
The code uses the shutil.copyfileobj() function to copy the contents of the source_file object to a new file called file.txt. The with statement is used to open and automatically close the file, using the file object f.
Time Complexity:
The time complexity of shutil.copyfileobj() function is proportional to the size of the file being copied, as it needs to read and write every byte of the file. Therefore, the time complexity of the code is O(n), where n is the size of the source_file.
Space Complexity:
The space complexity of the code is O(1), as it does not allocate any additional memory beyond what is required for the file objects source_file and f. The shutil.copyfileobj() function copies the file contents in chunks, so it does not need to load the entire file into memory at once.
Overall, the code has a linear time complexity and constant space complexity, where the time complexity is proportional to the size of the file being copied.