How to create python bytes object from long hex string?

Question

I am building an application on Rails where a user creates a Test, goes to the show view for that test and fills in a form with an answer to a question. If the answer to the question matches the "correct_answer" (defined in the controller), there will be flash message that the answer was correct and a button to continue to the root_path will appear. If the answer is wrong, the flash says "Wrong answer."

My problem is that the flash message will say wrong answer even if no answer has been given. I ONLY want that message to appear AFTER the user has submitted the form. I understand why this is happening, I just am unsure about how to fix it. Here is the show view for a test:

<div class="col-md-8 col-md-push-2">
        <h4>Current Score: <%= @test.score %></h4>
        <br /><br />


        <div class="form_group">
        <%= form_tag test_path(@test), :method=> 'get' do %>
            <h4>What is my first name?</h4>
            <div class="form-group">
                <%= text_field_tag :answer, params[:answer], class: 'form-control' %>
            </div>
            <% if !flash[:success] %>
            <div class="form-group">
                <%= submit_tag "Submit", class: "btn btn-primary" %>
            </div>
            <% end %>
        <% end %>
        </div>

        <% if flash[:success] %>
            <%= link_to "Continue", root_path, class: "btn btn-success pull-right" %> 
        <% end %> 
    </div>

Here is the controller for tests, which contains the offending show action:

 class TestsController < ApplicationController

def index 
    @test = Test.new 
    @tests = Test.all
end 

def show 
    @test = Test.find(params[:id])
    correct_answer = "jack"
    user_answer = params[:answer]
    if user_answer == correct_answer
        flash.now[:success] = "That is correct!"
        new_score = @test.score += 1
        @test.update(score: new_score)
    elsif params[:answer] != correct_answer
        flash.now[:danger] = "Wrong answer" 
    end 
end 

def create 
    @test = Test.create(test_params)
    if @test.save 
        redirect_to test_path(@test)
        flash[:success] = "Test created"
    else 
        flash[:danger] = "There was a problem"
        render "index" 
    end 
end 

def destroy 
    @test = Test.find(params[:id])
    if @test.destroy
        flash[:success] = "Your test was removed"
        redirect_to root_path 
    end 
end 

private 

    def test_params
        params.require(:test).permit(:score, :user_id)
    end
end

Is there a better way to do this? If not, can I somehow stop the flash message from appearing on the initial load? I ONLY want it to appear once the form has been submitted. Thanks in advance.

Solution 1

result = bytes.fromhex(some_hex_string)

Solution 2

Works in Python 2.7 and higher including python3:

result = bytearray.fromhex('deadbeef')

Note: There seems to be a bug with the bytearray.fromhex() function in Python 2.6. The python.org documentation states that the function accepts a string as an argument, but when applied, the following error is thrown:

>>> bytearray.fromhex('B9 01EF')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: fromhex() argument 1 must be unicode, not str`

Solution 3

You can do this with the hex codec. ie:

>>> s='000000000000484240FA063DE5D0B744ADBED63A81FAEA390000C8428640A43D5005BD44'
>>> s.decode('hex')
'\x00\x00\x00\x00\x00\x00HB@\xfa\x06=\xe5\xd0\xb7D\xad\xbe\xd6:\x81\xfa\xea9\x00\x00\xc8B\x86@\xa4=P\x05\xbdD'

Solution 4

Try the binascii module

from binascii import unhexlify
b = unhexlify(myhexstr)

Solution 5

import binascii

binascii.a2b_hex(hex_string)

Thats the way I did it.

View more solutions

Comments

[Paul Hoffman]

Note that the answers below may look alike but they return different types of values. s.decode('hex') returns a str, as does unhexlify(s). bytearray.fromhex(s) returns a bytearray. Given the wording of this question, I think the big green checkmark should be on bytearray.fromhex(s), not on s.decode('hex').

[Ciro Santilli OurBigBook.com]

Possible duplicate of hexadecimal string to byte array in python

[B Bulfin]

How can it be a duplicate of a question created 2 years later?

[Pavan]

Does the params[:answer] only available when you submit the form?

[Pavan]

Also can you update the question with the params that are generated in the server on the initial load?

[LarsH]

@CiroSantilli郝海东冠状病六四事件法轮功 A byte string isn't a byte array. stackoverflow.com/questions/1740696/…

[Ciro Santilli OurBigBook.com]

@LarsH fair enough. @ recursive: date is not the main factor: meta.stackexchange.com/questions/147643/…

[sath garcia]

Two ways to do it in 2.x, three ways in 3.x. So much for "there's only one way to do it"...

[Crescent Fresh]

Other two ways are more 'built-in' so I would actually use one of those.

[Ishbir]

@technomalogical: your comment is irrelevant to the answer; perhaps you should delete it and change it into a post to comp.lang.python .

[nosklo]

@technomalogical: I agree with ΤΖΩΤΖΙΟΥ. Also, you got it wrong. The correct phrase is: There should be one-- and preferably only one --obvious way to do it.

[Scott Griffiths]

Note that in Python 3.2 (whether by design or a bug I'm not sure) unhexlify now won't accept a string, but only bytes. Pretty silly really, but it means you'd need to use b = unhexlify(bytes(myhexstr, 'utf-8'))

[Abbafei]

codecs.decode('0a0a0a', 'hex_codec') should work for 2.x and 3.x :-)

[berto]

And one additional step, I wanted a byte string (e.g. Python 3's b'\x04\xea[...]'), which you can get from a bytearray with bytes(bytearray.fromhex('deadbeef'))

[Martijn Pieters]

@berto: in that case there is a more direct route in the form of binascii.unhexlify().

[berto]

Thanks, @MartijnPieters, I'll give that a shot

[Bash]

This seem like the most direct way to do what the original post is asking. Is there a reason this isn't the accepted answer?

[Klaws]

The fromhex() method (of both bytes and bytearray) will also work when the hex numbers are separated by spaces. Very convenient!

[Mike Martin]

This really should be the accepted answer. The current accepted answer does not do what the question asked. It returns a mutable array of bytes, not a bytestring.

[Mike Martin]

This answer does not do what the question asked. It returns a mutable array of bytes, not a python bytestring. That's like returning an array of strings rather than a string.

[LarsH]

@MartijnPieters Seems like bytes.fromhex() is also more direct, and a bit less obscure / more parsimonious. docs.python.org/3/library/stdtypes.html#bytes.fromhex

[Martijn Pieters]

@LarsH: that method is not available in older Python 2 releases. That doesn't matter any more today, but it was an issue in 2016.

[LarsH]

@MartijnPieters I wondered if that was the case, but I didn't look far enough to find out.

[B Bulfin]

@MikeMartin I accepted this answer, only 12 years after it was posted.