String formatting (% operator)

The function of formatting strings is included in many programming languages, such as formatted input and output in C and Fortran languages. The built-in operator for formatting strings in Python is "%".

Template

When formatting a string, Python uses a string as a template. Templates have formatters that reserve space for real values ​​and describe the format in which real values ​​should appear. Python uses a tuple to pass multiple values ​​to the template, each value corresponding to a format character.

For example, the following example:

print("I'm %s. I'm %d" % ('Pythontab', 1))

In the above example,

"I'm %s. I 'm %d" is our template. %s is the first format character, representing a string. %d is the second format character, representing an integer. The two elements 'Pythontab' and 99 of ('Pythontab',99) are the real values ​​that replace %s and %d.

Between the template and the tuple, there is a % sign separating it, which represents the formatting operation.

The entire "I'm %s. I'm %d" % ('Pythontab', 1) actually constitutes a string expression. We can assign it to a variable just like a normal string. For example:

a = "I'm %s. I'm %d" % ('Pythontab', 1)

print(a)

The result is: I'm Pythontab. I'm 1

us Dictionaries can also be used to pass real values. As follows:

print("I'm %(name)s. I'm %(age)d" % {'name':'Pythontab', 'age':99})

You can see that we Two format characters are named. Names are enclosed in (). Each name corresponds to a key in the dictionary.

Format Characters

Format characters reserve space for real values ​​and control the format of display. The format character can contain a type code to control the type of display, as follows:

%s String (displayed using str())

%r String (displayed using repr())

%c Single character

%b Binary integer

%d Decimal integer

%i Decimal integer

%o Octal integer

%x Hexadecimal integer

%e Exponent (base written as e)

%E Exponent (the base is written as E)

%f Floating point number

%F Floating point number, the same as above

%g Exponent (e) or floating point number (according to the display length)

%G Exponent (E) or floating point number (According to the display length)

%% The character "%"

can be further controlled in the format as follows:

%[(name)][flags][width].[precision]typecode

( name) is named

flags can have +, -, ' ' or 0. + means right alignment. - means left alignment. ' ' is a space, which means padding a space to the left of the positive number to align it with the negative number. 0 means use 0 padding.

width represents the display width

precision represents the precision after the decimal point

For example:

print("%+10x" % 10)

print("%04d" % 5)

print("%6.3f" % 2.3)

The width and precision above are two integers. We can use * to dynamically substitute these two quantities. For example:

print("%.*f" % (4, 1.2))

Python actually replaces * with 4. So the actual template is "%.4f".

Summary

The built-in % operator in Python can be used to format string operations and control the presentation format of strings. There are other ways to format strings in Python, but using the % operator is the most convenient.