<p>I'm using Python, and would like to insert a string into a text file without deleting or copying the file. How can I do that?</p>

Depends on what you want to do.  To append you can open it with "a":
```
```
with open("foo.txt", "a") as f:
     f.write("new line\n")
```
```
If you want to preprend something you have to read from the file first:
```
```
with open("foo.txt", "r+") as f:
     old = f.read() # read everything in the file
     f.seek(0) # rewind
     f.write("new line\n" + old) # write the new line before
```
```