Using struct.pack with strings

Question

I'm trying to use the struct.pack so I can write a string into a file. When I do, I get the following error:

File "----", line 166, in main
struct.pack('>256s', *master_header)
struct.error: pack expected 1 items for packing (got 256)

Now, reading from here I seem to be using it right. I specify that I'm getting 256 bytes/characters in my string.

I'm using version 3.3.3.

"so I can write a string into a file" - as in, you have a string, and you want to write it to a file, or you have something else, and you want to make it a string so you can write it to a file? — user2357112
– user2357112, Commented Feb 12, 2014 at 11:27
I have a 256 byte string I want to pack into a binary string using struct.pack. — zdc000
– zdc000, Commented Feb 12, 2014 at 22:24

user4815162342 · Accepted Answer · 2014-02-13 08:30:38Z

1

The documentation says:

For the 's' format character, the count is interpreted as the size of the string, not a repeat count like for the other format characters; for example, '10s' means a single 10-byte string, while '10c' means 10 characters.

So, >256s expects a single string 256 bytes long. If master_header already is such a string, just pass it to struct.pack without the *.

Using the * at the call site causes the string itself to be unpacked into its constituent characters, strings being iterable. As a result, struct.pack receives its 256 individual characters as arguments, causing the observed error.

edited Feb 13, 2014 at 8:30

answered Feb 12, 2014 at 11:28

user4815162342

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2 Comments

zdc000 Over a year ago

master_header is a single string that's 256 bytes long.

user4815162342 Over a year ago

@zdc000 In that case, just lose the *.

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