F-String was introduced in Python 3.6, and is now the preferred way of formatting strings.
Before Python 3.6 we had to use the format() method.
F-Strings
F-string allows you to format selected parts of a string.
To specify a string as an f-string, simply put an f in front of the string literal, like this:
Example
Create an f-string:
txt = f"The price is 49 dollars"
print(txt)
Placeholders and Modifiers
To format values in an f-string, add placeholders {}, a placeholder can contain variables, operations, functions, and modifiers to format the value.
Example
Add a placeholder for the price variable:
price = 59
txt = f"The price is {price} dollars"
print(txt)
A placeholder can also include a modifier to format the value.
A modifier is included by adding a colon : followed by a legal formatting type, like .2f which means fixed point number with 2 decimals:
Example
Display the price with 2 decimals:
price = 59
txt = f"The price is {price:.2f} dollars"
print(txt)
You can also format a value directly without keeping it in a variable:
Example
Display the value 95 with 2 decimals:
txt = f"The price is {95:.2f} dollars"
print(txt)
Perform Operations in F-Strings
You can perform Python operations inside the placeholders.
You can do math operations:
Example
Perform a math operation in the placeholder, and return the result:
txt = f"The price is {20 * 59} dollars"
print(txt)
You can perform math operations on variables:
Example
Add taxes before displaying the price:
price = 59
tax = 0.25
txt = f"The price is {price + (price * tax)} dollars"
print(txt)
You can perform if...else statements inside the placeholders:
Example
Return “Expensive” if the price is over 50, otherwise return “Cheap”:
price = 49
txt = f"It is very {'Expensive' if price>50 else 'Cheap'}"
print(txt)
Execute Functions in F-Strings
You can execute functions inside the placeholder:
Example
Use the string method upper()to convert a value into upper case letters:
fruit = "apples"
txt = f"I love {fruit.upper()}"
print(txt)
The function does not have to be a built-in Python method, you can create your own functions and use them:
Example
Create a function that converts feet into meters:
def myconverter(x):
return x * 0.3048
txt = f"The plane is flying at a {myconverter(30000)} meter altitude"
print(txt)
More Modifiers
At the beginning of this chapter we explained how to use the .2f modifier to format a number into a fixed point number with 2 decimals.
There are several other modifiers that can be used to format values:
Example
Use a comma as a thousand separator:
price = 59000
txt = f"The price is {price:,} dollars"
print(txt)
Here is a list of all the formatting types.
| Formatting Types | ||
|---|---|---|
:< |
||
| Left aligns the result (within the available space) | |
:> |
| Right aligns the result (within the available space) | |
:^ |
| Center aligns the result (within the available space) | |
:= |
| Places the sign to the left most position | |
:+ |
| Use a plus sign to indicate if the result is positive or negative | |
:- |
| Use a minus sign for negative values only | |
: |
| Use a space to insert an extra space before positive numbers (and a minus sign before negative numbers) | |
:, |
| Use a comma as a thousand separator | |
:_ |
| Use a underscore as a thousand separator | |
:b |
| Binary format | ||
:c |
Converts the value into the corresponding Unicode character | |
:d |
| Decimal format | |
:e |
| Scientific format, with a lower case e | |
:E |
| Scientific format, with an upper case E | |
:f |
| Fix point number format | |
:F |
Fix point number format, in uppercase format (show inf and nan as INF and NAN) |
||
:g |
General format | |
:G |
General format (using a upper case E for scientific notations) | |
:o |
| Octal format | |
:x |
| Hex format, lower case | |
:X |
| Hex format, upper case | ||
:n |
Number format | |
:% |
| Percentage format |
String format()
Before Python 3.6 we used the format() method to format strings.
The format() method can still be used, but f-strings are faster and the preferred way to format strings.
The next examples in this page demonstrates how to format strings with the format() method.
The format() method also uses curly brackets as placeholders {}, but the syntax is slightly different:
Example
Add a placeholder where you want to display the price:
price = 49
txt = "The price is {} dollars"
print(txt.format(price))
You can add parameters inside the curly brackets to specify how to convert the value:
Example
Format the price to be displayed as a number with two decimals:
txt = "The price is {:.2f} dollars"
Check out all formatting types in our String format() Reference.
Multiple Values
If you want to use more values, just add more values to the format() method:
print(txt.format(price, itemno, count))
And add more placeholders:
Example
quantity = 3
itemno = 567
price = 49
myorder = "I want {} pieces of item number {} for {:.2f} dollars."
print(myorder.format(quantity, itemno, price))
Index Numbers
You can use index numbers (a number inside the curly brackets {0}) to be sure the values are placed in the correct placeholders:
Example
quantity = 3
itemno = 567
price = 49
myorder = "I want {0} pieces of item number {1} for {2:.2f} dollars."
print(myorder.format(quantity, itemno, price))
Also, if you want to refer to the same value more than once, use the index number:
Example
age = 36
name = "John"
txt = "His name is {1}. {1} is {0} years old."
print(txt.format(age, name))
Named Indexes
You can also use named indexes by entering a name inside the curly brackets {carname}, but then you must use names when you pass the parameter values txt.format(carname = "Ford"):
Example
myorder = "I have a {carname}, it is a {model}."
print(myorder.format(carname = "Ford", model = "Mustang"))