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convert an negative integer to 32bit binary in python

I would like to create a function which gives the following things
32_bit_binary(-1) should be '11111111111111111111111111111111'
32_bit_binary(1) should be '00000000000000000000000000000001'
Now I gave the following code
def 32_bit_binary(num_bits):
return '{:032b}'.format(num_bits)
But when I gave this
print(32_bit_binary(-1))
it came -00000000000000000000000000000001
What is wrong with the code?

As #gspr said, formatting as base 2 doesn’t give you the actual representation. You can solve it by masking the negative integer, which has infinitely many leading 1s for the purposes of bitwise operations, down to 32 bits:
return f"{num_bits & 0xffff_ffff:032b}"

String formatting like {:032b} does not give you the actual representation of the number. It just writes the number in base-2. That's a purely mathematical operation. Implementation details like how the computer represents said number (like using binary, using 2's complement for negative numbers, etc.) are not in the scope of those string formatting operations.
A good way to get at the actual representations of values in Python is the struct module. For example, struct.pack("#i", -1) returns the bytestring b'\xff\xff\xff\xff'. Printing that bytestring in binary is left as an exercise to the reader.
PS: For numbers other than -1, you may be surprised by the output of struct.pack. The term you'll want to look up is endianness, and the # in my struct.pack formatting string.

Convert float to Decimal with fixed digits after decimal

I want to convert some floats to Decimal retaining 5 digits after decimal place regardless of how many digits before the decimal place. Is using string formatting the most efficient way to do this?
I see in the docs:
The significance of a new Decimal is determined solely by the number of digits input. Context precision and rounding only come into play during arithmetic operations.
So that means I need to add 0 to force it to use the specified prec but the prec is total digits not after decimal so it doesn't actually help.
The best thing I can come up with is
a=[1.132434, 22.2334,99.33999434]
[Decimal("%.5f" % round(x,5)) for x in a]
to get [Decimal('1.13243'), Decimal('22.23340'), Decimal('99.33999')]
Is there a better way? It feels like turning floats into strings just to convert them back to a number format isn't very good although I can't articulate why.

Do all the formatting on the way out from your code, inside the print and write statements. There is no reason I can think of to lose precision (and convert the numbers to some fixed format) while doing numeric calculations inside the code.

How to convert all floats to two decimal places regardless of number of decimal places float has without repeating convert code?

Goal
I want to convert all floats to two decimal places regardless of number of decimal places float has without repeating convert code.
For example, I want to convert
50 to 50.00
50.5 to 50.50
without repeating the convert code again and again. What I mean is explained in the following section - research.
Not what this question is about
This question is NOT about:
Only limiting floats to two decimal points - if there is less than two decimal points then I want it to have two decimal points with zeros for the unused spaces.
Flooring the decimal or ceiling it.
Rounding the decimal off.
This question is not a duplicate of this question.
That question only answers the first part of my question - convert floats to two decimal places regardless of number of decimal places float has, not the second part - without repeating convert code.
Nor this question.
That is just how to add units before the decimal place. My question
is: how to convert all floats to two decimal places regardless of
number of decimal places float has without repeating convert code.
Research
I found two ways I can achieve the convert. One is using the decimal module:
from decimal import *
TWOPLACES = Decimal(10) ** -2
print(Decimal('9.9').quantize(TWOPLACES))
Another, without using any other modules:
print(f"{9.9:.2f}")
However, that does not fully answer my question. Realise that the code to convert keeps being needed to repeat itself? I keep having to repeat the code to convert again and again. Sadly, my whole program is already almost completed and it will be quite a waste of time to add this code here and there so the format will be correct. Is there any way to convert all floats to two decimal places regardless of number of decimal places float has without repeating convert code?
Clarification
What I mean by convert is, something like what Dmytro Chasovskyi said, that I want all places with floats in my program without extra changes to start to operate like decimals. For example, if I had the operation 1.2345 + 2.7 + 3 + 56.1223183 it should be 1.23 + 2.70 + 3.00 + 56.12.
Also, float is a number, not a function.

The bad news is: there is no "float" with "two decimal places".
Floating point numbers are represented internally with a fixed number of digits in base 2. https://floating-point-gui.de/basic/ .
And these are both efficient and accurate enough for almost all calculations we perform with any modern program.
What we normally want is that the human-readable text representation of a number, in all outputs of a program, shows only two digits. And this is controlled at wherever your program is either writting the value to a text file, to the screen, or rendering it to an HTML template (which is "writing it to a text file", again).
So, it happens that the same syntaxes that will convert a number to text, embedded in another string, allows additionally to control the exact output of the number. You put as an example print(f"{9.9:.2f}"). The only thing that looks impractical there is due to you hardcoding your number along with its conversion. Typically, the number will be in a variable.
Them, all you have to do is writting, wherever you output the number:
print(f"The value is: {myvar:.02f}")
instead of
print(f"The value is: {myvar}")
Or in whatever function you are calling that will need the rendered version of the number instead of print. Notice that the use of the word "rendered" here is deliberate: while your program is running, the number is stored in an efficient way in memory, directly usable by the CPU, that is not human readable. At any point you want to "see" the number, you have to convert it into text. It is just that some calls to it implicitly, like print(myvar). Then, just resort to explicitly converting it in these places - `print(f"{myvar:.02f}").
really having 2 decimal places in memory
If you use decimal.Decimal, then yes, there are ways to keep the internal representation of the number with 2 decimal digits,
but them, instead of just converting the number on output, you must convert it into a "2 decimal place" value on all inputs as well
That means that whenever ingesting a number into your program, be it typed by the user, read from a binary file or database, or received via wire from a sensor, you have to apply a similar transform to the one used in the output as detailed above. More precisely: you convert your float to a properly formatted string, and then convert that to a decimal.Decimal.
And this will prevent your program of accumulating errors due to base conversion, but you will still need to force the format to 2 decimal places on every output, just like above.

Use this function.
def cvt_decimal(input):
number = float(input)
return ("%.2f" % number)
print(cvt_decimal(50))
print(cvt_decimal(50.5))
Output is :
50.00
50.50
** Process exited - Return Code: 0 **
Press Enter to exit terminal

you can modify the decimal precision, even if you do any operation between 2 decimal types
import decimal
from decimal import Decimal
decimal.getcontext().prec = 2
a = Decimal('0.12345')
b = Decimal('0.12345')
print(a + b)
Decimal calculations are precise but it takes more time to do calculations, keep that in mind.

Python default behavior of str(x)

I am depending on some code that uses the Decimal class because it needs precision to a certain number of decimal places. Some of the functions allow inputs to be floats because of the way that it interfaces with other parts of the codebase. To convert them to decimal objects, it uses things like
mydec = decimal.Decimal(str(x))
where x is the float taken as input. My question is, does anyone know what the standard is for the 'str' method as applied to floats?
For example, take the number 2.1234512. It is stored internally as 2.12345119999999999 because of how floats are represented.
>>> x = 2.12345119999999999
>>> x
2.1234511999999999
>>> str(x)
'2.1234512'
Ok, str(x) in this case is doing something like '%.6f' % x. This is a problem with the way my code converts to decimals. Take the following:
>>> d = decimal.Decimal('2.12345119999999999')
>>> ds = decimal.Decimal(str(2.12345119999999999))
>>> d - ds
Decimal('-1E-17')
So if I have the float, 2.12345119999999999, and I want to pass it to Decimal, converting it to a string using str() gets me the wrong answer. I need to know what are the rules for str(x) that determine what the formatting will be, because I need to determine whether this code needs to be re-written to avoid this error (note that it might be OK, because, for example, the code might round to the 10th decimal place once we have a decimal object)
There must be some set of rules in python's docs that hopefully someone here can point me to. Thanks!

In the Python source, look in "Include/floatobject.h". The precision for the string conversion is set a few lines from the top after an comment with some explanation of the choice:
/* The str() precision PyFloat_STR_PRECISION is chosen so that in most cases,
the rounding noise created by various operations is suppressed, while
giving plenty of precision for practical use. */
#define PyFloat_STR_PRECISION 12
You have the option of rebuilding, if you need something different. Any changes will change formatting of floats and complex numbers. See ./Objects/complexobject.c and ./Objects/floatobject.c. Also, you can compare the difference between how repr and str convert doubles in these two files.

There's a couple of issues worth discussing here, but the summary is: you cannot extract information that is not stored on your system already.
If you've taken a decimal number and stored it as a floating point, you'll have lost information, since most decimal (base 10) numbers with a finite number of digits cannot be stored using a finite number of digits in base 2 (binary).
As was mentioned, str(a_float) will really call a_float.__str__(). As the documentation states, the purpose of that method is to
return a string containing a nicely printable representation of an object
There's no particular definition for the float case. My opinion is that, for your purposes, you should consider __str__'s behavior to be undefined, since there's no official documentation on it - the current implementation can change anytime.
If you don't have the original strings, there's no way to extract the missing digits of the decimal representation from the float objects. All you can do is round predictably, using string formatting (which you mention):
Decimal( "{0:.5f}".format(a_float) )
You can also remove 0s on the right with resulting_string.rstrip("0").
Again, this method does not recover the information that has been lost.

How do I force Python to keep an integer out of scientific notation

I am trying to write a method in Python 3.2 that encrypts a phrase and then decrypts it. The problem is that the numbers are so big that when Python does math with them it immediately converts it into scientific notation. Since my code requires all the numbers to function scientific notation, this is not useful.
What I have is:
coded = ((eval(input(':'))+1213633288469888484)/2)+1042
Basically, I just get a number from the user and do some math to it.
I have tried format() and a couple other things but I can't get them to work.
EDIT: I use only even integers.

In python3, '/' does real division (e.g. floating point). To get integer division, you need to use //. In other words 100/2 yields 50.0 (float) whereas 100//2 yields 50 (integer)
Your code probably needs to be changed as:
coded = ((eval(input(':'))+1213633288469888484)//2)+1042
As a cautionary tale however, you may want to consider using int instead of eval:
coded = ((int(input(':'))+1213633288469888484)//2)+1042

If you know that the floating point value is really an integer, or you don't care about dropping the fractional part, you can just convert it to an int before you print it.
>>> print 1.2e16
1.2e+16
>>> print int(1.2e16)
12000000000000000