SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes

The error

$ python notes.py
  File "notes.py", line 1
    path = "C:\Users\you\notes.txt"
           ^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 2-3: truncated \UXXXXXXXX escape

With a lower-case "C:\users\you" the message ends in truncated \uXXXX escape instead.

Why it happens

In an ordinary Python string a backslash starts an escape. \U must be followed by eight hex digits and \u by four, to name a Unicode character. sers isn't hex, so Python can't compile the file, and none of it runs, not even the lines above.

Coding agents hit it when they write a script into a Bash heredoc or python -c with a path copied from Windows.

The quiet version

A path without \U or \u compiles, but its other escapes still turn into characters:

$ python data.py
Traceback (most recent call last):
  File "data.py", line 1, in <module>
    f = open("C:\temp\new.csv")
OSError: [Errno 22] Invalid argument: 'C:\temp\new.csv'

\t became a tab and \n a newline. The message prints them back as \t and \n, so the path in it looks right. Escapes Python doesn't know, such as \d, give a SyntaxWarning: invalid escape sequence instead.

The fix

Any one of these keeps the backslashes, or does without them:

path = r"C:\Users\you\notes.txt"     # a raw string: backslashes stay as they are
path = "C:/Users/you/notes.txt"      # Windows takes forward slashes
path = "C:\\Users\\you\\notes.txt"   # each backslash doubled

Or build the path, and skip the question:

from pathlib import Path
path = Path.home() / "notes.txt"

A raw string can't end in a single backslash: r"C:\Users\you\" is SyntaxError: unterminated string literal. Leave the last backslash off, or use forward slashes.

When an agent writes the script, have it write a .py file and run that, rather than pass the code through a shell, which can mangle it on the way (unterminated string literal).