Python: Format String

By Xah Lee. Date: . Last updated: .

Note: in practice, these two returns the same thing.

# -*- coding: utf-8 -*-
# python

ss = [3, 4, 3.123456789012345678901234567890, (7, 8, "9")]
print (str(ss))    # [3, 4, 3.1234567890123457, (7, 8, '9')]
print (repr(ss))   # [3, 4, 3.1234567890123457, (7, 8, '9')]

C Style printf

The “print” function itself supports string formatting in the style of C's printf. The syntax is: print "…" % (arg1, arg2, …)

#-*- coding: utf-8 -*-
# python

# integer
print "%d" % (1234)   # 「1234」

# padding by space
print "%4d" % (12)    # 「  12」

# float. 2 integer, 4 decimal
print "%2.4f" % (3.123456789)   # 「3.1235」

# string.
print "%5s" % ("cats")     # 「 cats」
print "%2s" % ("cats")     # 「cats」

print "%2d◇%6d◇%2.4f◇%s" % (1234, 5678, 3.123456789, 'cats!') # 「1234◇  5678◇3.1235◇cats!」

Python Style String 「.format()」 Method

A better way to format string is str.format(arg1, arg2, …).

Use {…} as placeholder in string.

# -*- coding: utf-8 -*-
# python

# “:s” is for string
print "{:s}".format("cat")     # cat

# “:s” is default, and can be omitted.
print "{}".format([3, 4])     # [3, 4]

# decimal int
print "{:d}".format(3)     # 3

# binary
print "{:b}".format(3)     # 11

# hex
print "{:x}".format(10)     # a

# formatting multiple args
print "{:d}, {:s}, {:s}".format(3, "cat", [7, 8])

# use {{}} to include literal {}
print "brace {{}}, a is {}".format(3)     # brace {}, a is 3

You can refer to the argument and change print order.

# -*- coding: utf-8 -*-
# python

print "{0:d}, {1:s}, {2:s}".format(3, "cat", [7, 8]) # 3, cat, [7, 8]

print "{1:s}, {0:d}, {2:s}".format(3, "cat", [7, 8]) # cat, 3, [7, 8]

Printing Without Newline

To print without the newline char added at the end, add a comma. Example:

# -*- coding: utf-8 -*-
# python

print "rabbit",  # suppress printing newline
print "tiger"

# prints 「rabbit tiger」

Or, use sys.stdout.write(). Example:

# -*- coding: utf-8 -*-
# python

import sys
sys.stdout.write("rabbit")
sys.stdout.write("tiger")

# output: 「rabbittiger」

See also: Python 3: Formatting String

Thanks to Yuri Khan https://plus.google.com/109564459582005085765/posts for a suggetion on Python print.

Python String

  1. Quote String
  2. String Operations
  3. String Methods
  4. Format String

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Python by Example

  1. Python Basics
  2. Print Version String
  3. Builtin Help
  4. Quote String
  5. String Operations
  6. String Methods
  7. Format String
  8. True, False
  9. if then else
  10. for, while, Loops
  11. List Basics
  12. Loop Thru List
  13. Map Function to List
  14. List Comprehension
  15. List Methods
  16. Dictionary
  17. Loop Thru Dict
  18. Dict Methods
  19. Function
  20. Class
  21. List Modules
  22. Write a Module
  23. Unicode 🐍

Regex

  1. Regex Basics
  2. Regex Reference

Text Processing

  1. Read/Write File
  2. Traverse Directory
  3. Manipulate Path
  4. Process Unicode
  5. Convert File Encoding
  6. Find Replace in dir
  7. Find Replace by Regex
  8. Count Word Frequency

Web

  1. Send Email
  2. GET Web Page
  3. Web Crawler
  4. HTTP POST
  5. Check Page Load Size
  6. Thumbnail Generation

Misc

  1. JSON
  2. Find Script Path
  3. Get Env Var
  4. System Call
  5. Decompress Gzip
  6. Complex Numbers

Advanced

  1. Sort
  2. Copy Nested List
  3. Tuple vs List
  4. Sets, Union, Intersection
  5. Closure in Python 2
  6. Decorator
  7. Append String in Loop
  8. Timing f timeit
  9. Keyword Arg Default Value Unstable

Reference