[Python] Why does file.write() in "r+" mode not overwrite old content as expected?

I expected the following Python code to overwrite part of the file:

#PYTHON
# File "rplus.txt" initially contains: Hello, world!
with open("rplus.txt", "r+") as file:
print(file.read(5)) # Outputs: Hello
file.write(", Python!") # Attempt to overwrite the rest
file.seek(0)
print(file.read()) # Unexpected output



But instead of getting:

Hello

Hello, Python!

I got:

Hello

Hello, world!, Python!

I thought file.write() in "r+" mode would start writing at the current cursor and overwrite existing characters. Why is the original "world!" still there? Shouldn't it be overwritten?

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