Add zeros to a float after the decimal point in Python

Question

I am reading in data from a file, modify it and write it to another file. The new file will be read by another program and therefore it is crucial to carry over the exact formatting

for example, one of the numbers on my input file is:

1.000000

my script applies some math to the columns and should return

2.000000

But what is currently returned is

2.0

How would I write a float for example my_float = 2.0, as my_float = 2.00000 to a file?

Solution 1

Format it to 6 decimal places:

format(value, '.6f')

Demo:

>>> format(2.0, '.6f')
'2.000000'

The format() function turns values to strings following the formatting instructions given.

Solution 2

From Python 3.6 it's also possible to do f-string formatting. This looks like:

f"{value:.6f}"

Example:

> print(f"{2.0:.6f}")
'2.000000'

Solution 3

I've tried n ways but nothing worked that way I was wanting in, at last, this worked for me.

foo = 56
print (format(foo, '.1f'))
print (format(foo, '.2f'))
print (format(foo, '.3f'))
print (format(foo, '.5f'))

output:

56.0 
56.00
56.000
56.00000

Meaning that the 2nd argument of format takes the decimal places you'd have to go up to. Keep in mind that format returns string.

Solution 4

I've had problems with using variables in f strings. When all else fails, read the manual :)

"A consequence of sharing the same syntax as regular string literals is that characters in the replacement fields must not conflict with the quoting used in the outer formatted string literal."

https://docs.python.org/3/reference/lexical_analysis.html#f-strings

Case in point:

my_number = 90000
zeros = '.2f'

my_string = f"{my_number:,{zeros}}"

print (my_string)
90,000.00

my_string = f'{my_number:,{zeros}}'

will not work, because of the single quotes.

Quotes containing the f string and the string variable used in the f string should be different.

If using single quotes for the string variable, use double quotes for the f module and vice versa.

Solution 5

An answer using the format() command is above, but you may want to look into the Decimal standard library object if you're working with floats that need to represent an exact value. You can set the precision and rounding in its context class, but by default it will retain the number of zeros you place into it:

>>> import decimal
>>> x = decimal.Decimal('2.0000')
>>> x
Decimal('2.0000')
>>> print x
2.0000
>>> print "{0} is a great number.".format(x)
2.0000 is a great number.

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Comments

[Martijn Pieters]

@DemianBrecht: We are talking about formatting floats here.

[Demian Brecht]

@MartijnPieters: Whether formatting a float or formatting a string representation of a float, the output is the same when writing to a file. Although I agree (now) that my link is not a dupe, but perhaps "related".

[Justin Carroll]

Correct me if I am wrong, but this also has the implicit instruction of "round to 6 decimal places" not just "keep 6 decimal places". So while it makes sense in the context of all zeros, the reason one might go to the precision is implied that you may actually USE that precision, as such you may need to go out further (and truncate) if you want to avoid rounding errors). For example, try: format(2.0000008, '.6f'). But otherwise, I totally agree with Martijn.

[Martijn Pieters]

@Nascent_Notes: Since the input is limited to 6 decimal places, I think it is safe to assume that rounding to 6 decimals is fine.

[Admin]

Thanks, that solution works totally fine for me! And yes, the input is limited to 6 decimal places so I (hopefully) wouldn't bump into rounding errors

[michaelmoo]

@JustinCarroll I don't understand what you're getting at, so in case anyone else is confused: the format command shown by Martijn rounds the value. format( 5.555, '.2f' ) gives 5.55. format( 5.5551, '.2f' ) gives 5.56. format(2.0000008, '.6f') gives 2.000001.

[Justin Carroll]

@michaelmoo, my comment was just that. Format does rounding, not just padding. IFF, the question read "how to pad with 5's", you couldn't do format(5.55555, '.4f'). That would give the result 5.5556. It was simple a warning. I think your comment is saying the same thing, just phrased alternatively.

[jeromej]

How do you set the precision to something else?

[Wes]

Here's a link to the format spec mini language: docs.python.org/2/library/string.html#formatspec