io.BytesIO does not have peek()

@benjaminp

BPO	46375
Nosy	@benjaminp, @marcelm, @kumaraditya303
PRs	gh-90533: Implement BytesIO.peek() #30808

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-01-14.10:39:24.835>
labels = ['type-feature', 'library', '3.11']
title = 'io.BytesIO does not have peek()'
updated_at = <Date 2022-01-22.22:08:29.533>
user = 'https://github.com/marcelm'

bugs.python.org fields:

activity = <Date 2022-01-22.22:08:29.533>
actor = 'marcelm'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-01-14.10:39:24.835>
creator = 'marcelm'
dependencies = []
files = []
hgrepos = []
issue_num = 46375
keywords = ['patch']
message_count = 2.0
messages = ['410552', '411294']
nosy_count = 4.0
nosy_names = ['benjamin.peterson', 'stutzbach', 'marcelm', 'kumaraditya']
pr_nums = ['30808']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46375'
versions = ['Python 3.11']

Linked PRs

gh-90533: Implement BytesIO.peek() #150917

It would be great to be able to use peek() on BytesIO objects.

I have a function that gets passed a file-like object and uses peek() on it. This works for nearly all types of files relevant in my library, except BytesIO instances (which I use during testing), for which I need to add a small workaround using tell() and seek().

I opened a PR, but I now wonder whether the missing peek() is by design.

First, I noticed that instead of using BytesIO directly, I can wrap the instance in an io.BufferedReader, which does have peek(). (It’s just a bit inconvenient.)

The second thing is that BytesIO is currently documented to inherit from BufferedIOBase, but if peek() is implemented, one could argue that BytesIO now should inherit from BufferedReader because it then has all the methods. And that seems to great a change from my perspective.

I’ll defer to someone more knowledgeable and do not mind at all if this issue is closed without action.

Correct me if I am wrong, but I think you want a io.BufferedRandom object? I am assuming you reached first for io.BytesIO, as I did, and found it was missing the peek function.

You are correct in that BufferedReader supports peek - but this leads to a slightly strange asymmetrical design. I assume you wanted to write to this buffer as well as read from it?

Assuming your situation is the same as mine - the right tool to use is io.BufferedRandom. The slightly unusual name I suppose obscures its intended use.

Hope that helps you? Of course feel free to ignore my comment if your situation is not as I have described.

Best wishes

gh-90533: Implement BytesIO.peek() (#150917)

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io.BytesIO does not have peek() #90533

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io.BytesIO does not have peek() #90533

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marcelm commented on Jan 14, 2022

marcelm commented on Jan 22, 2022

edward-b-1 commented on Dec 1, 2024

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