There are two string formatting schemes in Python, one is the % operator that existed before Python 2.6, and the other is a new string formatting function str.format() that has been added since Python 2.6.

% Formatting Tool

Students familiar with the C printf() function will easily learn the %formatting tool. print() uses conversion specifiers beginning with % to format output for various types of data.

Output of integers.

  • %o – octal octal
  • %d – dec decimal
  • %x – hex hexadecimal

Output of floating point numbers.

  • %f – retains six significant digits after the decimal point. %.3f, retains 3 decimal places
  • %e – retains six significant digits after the decimal point, output in exponential form. %.3e, retains three decimal places, uses scientific notation
  • %g – use decimal form if six valid digits are guaranteed, otherwise use scientific notation %.3g, retaining 3 significant digits, using decimal or scientific notation

String output.

  • %s – output string
  • %10s – right justified, placeholder 10 bits
  • %-10s – left justified, placeholder 10 bits
  • %.2s – intercept 2-bit string
  • %10.2s – 10-bit placeholder, two-bit string intercepted
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print('%o' % 20)
print('%d' % 20)
print('%x' % 20)

print('%f' % 1.11)  # 默认保留6位小数
print('%.1f' % 1.11)  # 取1位小数
print('%e' % 1.11)  # 默认6位小数，用科学计数法
print('%.3e' % 1.11)  # 取3位小数，用科学计数法
print('%g' % 1111.1111)  # 默认6位有效数字
print('%.7g' % 1111.1111)  # 取7位有效数字
print('%.2g' % 1111.1111)  # 取2位有效数字，自动转换为科学计数法

print('%s' % 'hello world')  # 字符串输出
print('%20s' % 'hello world')  # 右对齐，取20位，不够则补位
print('%-20s' % 'hello world')  # 左对齐，取20位，不够则补位
print('%.2s' % 'hello world')  # 取2位
print('%10.2s' % 'hello world')  # 右对齐，取2位
print('%-10.2s' % 'hello world')  # 左对齐，取2位

# 输出：
# 24
# 20
# 14
# 1.110000
# 1.1
# 1.110000e+00
# 1.110e+00
# 1111.11
# 1111.111
# 1.1e+03
# hello world
#          hello world
# hello world         
# he
#         he
# he

format function

str.format() enhances string formatting. The basic syntax is via {} and : instead of the previous %. format function can take unlimited arguments and positions can be out of order.

Positional matching.

  • without numbering, i.e. “{}”
  • with numbering, i.e. “{1}”, “{2}”
  • with keywords, i.e. “{a}”, “{tom}”

Format conversion.

  • ‘b’ - binary. Outputs the number in base 2.
  • ‘c’ - Character. Converts integers to their corresponding Unicode strings before printing.
  • ’d’ - Decimal integer. Outputs the number in base 10.
  • ‘o’ - Octal. Outputs the number in base 8.
  • ‘x’ - Hexadecimal. Outputs numbers in base 16, with lowercase letters for digits above 9.
  • ’e’ - Power notation. Print numbers in scientific notation. Use ’e’ for powers.
  • ‘g’ - General format. Outputs the value in fixed-point format. When the value is particularly large, print it in power format.
  • ’n’ - Numeric. Same as ’d’ when the value is an integer, same as ‘g’ when the value is a floating-point number. The difference is that it inserts a number separator depending on the region setting.
  • ‘%’ - percent. Multiplies the value by 100 and prints it in fixed-point(‘f’) format, the value will be followed by a percent sign.

Left-center-right alignment and bit-completion.

  • < (default) left-aligned, > right-aligned, ^ middle-aligned, = (for numbers only) to round up after the decimal point
  • Take digits “{:4s}”, “{:.2f}”, etc.

Code examples and more.

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import datetime

print('{} {}'.format('hello', 'world'))  # 不带字段
print('{0} {1}'.format('hello', 'world'))  # 带数字编号
print('{0} {1} {0}'.format('hello', 'world'))  # 打乱顺序
print('{1} {1} {0}'.format('hello', 'world'))
print('{a} {tom} {a}'.format(tom='hello', a='world'))  # 带关键字

print('{0:b}'.format(3))
print('{:c}'.format(20))
print('{:d}'.format(20))
print('{:o}'.format(20))
print('{:x}'.format(20))
print('{:e}'.format(20))
print('{:g}'.format(20.1))
print('{:f}'.format(20))
print('{:n}'.format(20))
print('{:%}'.format(20))

print('{} and {}'.format('hello', 'world'))  # 默认左对齐
print('{:10s} and {:>10s}'.format('hello', 'world'))  # 取10位左对齐，取10位右对齐
print('{:^10s} and {:^10s}'.format('hello', 'world'))  # 取10位中间对齐
print('{} is {:.2f}'.format(1.123, 1.123))  # 取2位小数
print('{0} is {0:>10.2f}'.format(1.123))  # 取2位小数，右对齐，取10位
print('{:<30}'.format('left aligned'))  # 左对齐
print('{:>30}'.format('right aligned'))  # 右对齐
print('{:^30}'.format('centered'))  # 中间对齐
print('{:*^30}'.format('centered'))  # 使用“*”填充
print('{:0=30}'.format(11))  # 还有“=”只能应用于数字，这种方法可用“>”代替

print('{:+f}; {:+f}'.format(3.14, -3.14))  # 总是显示符号
print('{: f}; {: f}'.format(3.14, -3.14))  # 若是+数，则在前面留空格
print('{:-f}; {:-f}'.format(3.14, -3.14))  # -数时显示-，与'{:f}; {:f}'一致

print('{:,}'.format(1234567890))  # 数字千位分隔符

print('{:%Y-%m-%d %H:%M:%S}'.format(datetime.date.today()))  # 日期时间格式化

String interpolation F-strings

In Python version 3.6.2, PEP 498 introduced a new string formatting mechanism, called “string interpolation” or more commonly known as F-strings, which provides an explicit and convenient way to embed python expressions into strings to format them.

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# Python程序演示 
# 字符串插值 
n1 = 'Hello'
n2 ='GeeksforGeeks'
# f告诉Python恢复大括号{}中两个字符串变量名和程序的值
print(f"{n1}! This is {n2}")
# 内联算法 
print(f"(2 * 3)-10 = {(2 * 3)-10}")

To create an f string, prefix the string with the letter " f". The string itself is formatted as str.format(). f strings provide a concise and convenient way to format python expressions embedded in string literals.

Extension: String Templates

Template, set a string as a template, and finally get the desired string by replacing variables.

Usage examples.

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from string import Template

template_string = '$who likes $what'
s = Template(template_string)
d = {'who': 'Tim', 'what': 'Kong Fu'}
s_out = s.substitute(d)
print(s_out)