Here's an example using the format() function:

>>> "{:.2E}".format(Decimal('40800000000.00000000000000'))
'4.08E+10'

Instead of format, you can also use f-strings:

>>> f"{Decimal('40800000000.00000000000000'):.2E}"
'4.08E+10'

• official documentation

• original format() proposal

This is a consolidated list of the "Simple" Answers & Comments.

PYTHON 3

from decimal import Decimal

x = '40800000000.00000000000000'
# Converted to Float
x = Decimal(x)

# ===================================== # `Dot Format`
print("{0:.2E}".format(x))
# ===================================== # `%` Format
print("%.2E" % x)
# ===================================== # `f` Format
print(f"{x:.2E}")
# =====================================
# ALL Return: 4.08E+10
print((f"{x:.2E}") == ("%.2E" % x) == ("{0:.2E}".format(x)))
# True
print(type(f"{x:.2E}") == type("%.2E" % x) == type("{0:.2E}".format(x)))
# True
# =====================================

OR Without IMPORT's

# NO IMPORT NEEDED FOR BASIC FLOATS
y = '40800000000.00000000000000'
y = float(y)

# ===================================== # `Dot Format`
print("{0:.2E}".format(y))
# ===================================== # `%` Format
print("%.2E" % y)
# ===================================== # `f` Format
print(f"{y:.2E}")
# =====================================
# ALL Return: 4.08E+10
print((f"{y:.2E}") == ("%.2E" % y) == ("{0:.2E}".format(y)))
# True
print(type(f"{y:.2E}") == type("%.2E" % y) == type("{0:.2E}".format(y)))
# True
# =====================================

Comparing

# =====================================
x
# Decimal('40800000000.00000000000000')
y
# 40800000000.0

type(x)
# <class 'decimal.Decimal'>
type(y)
# <class 'float'>

x == y
# True
type(x) == type(y)
# False

x
# Decimal('40800000000.00000000000000')
y
# 40800000000.0

So for Python 3, you can switch between any of the three for now.

My Fav:

print("{0:.2E}".format(y))

I prefer Python 3.x way.

cal = 123.4567
print(f"result {cal:.4E}")

4 indicates how many digits are shown shown in the floating part.

cal = 123.4567
totalDigitInFloatingPArt = 4
print(f"result {cal:.{totalDigitInFloatingPArt}E} ")

Here is the simplest one I could find.

format(40800000000.00000000000000, '.2E')
#'4.08E+10'

('E' is not case sensitive. You can also use '.2e')

Adding an updated answer to show how to apply e notation to small numbers only

value = 0.1
a = "{:,}".format(value) if value >= 0.001 else "{:,.3e}".format(value)
print(a) # 0.1

value = 0.00002488
a = "{:,}".format(value) if value >= 0.001 else "{:,.3e}".format(value)
print(a) # 2.488e-05