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Generate randomized strings of characters using a template

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Generate test data, unique ids, passwords, vouchers or other randomized textual data very quickly using a template language. The template language is superficially similar to regular expressions but instead of defining how to match or capture strings, it defines how to generate randomized strings. A very simple invocation to produce a random string with word characters and digits of 10 characters length:

>>> import strgen
>>> strgen.StringGenerator("[\d\w]{10}").render()

The purpose of this module is to save the Python developer from having to write verbose code around the same pattern every time to generate passwords, keys, tokens, test data, etc. of this sort:

my_secret_key = ''.join(random.choice(string.ascii_uppercase + string.digits) for x in range(30))

that is:

  1. Hard to read even at this simplistic level.

  2. Hard to safely change quickly. Even modest additions to the requirements need unreasonably verbose solutions.

  3. Doesn’t use safe encryption standards.

  4. Doesn’t provide the implied minimal guarantees of character occurance.

  5. Hard to track back to requirements (“must be between x and y in length and have characters from sets Q, R and S”).

The template uses short forms similar to those of regular expressions. An example template for generating a strong password:


will generate something like the following:


Guarantee at least two “special” characters in a string:


You can also generate useful test data, like fake emails with plenty of variation:



Install as standard for Python packages from PyPi:

pip install StringGenerator


from strgen import StringGenerator

or to produce a list of unique strings:

from strgen import StringGenerator


>>> from strgen import StringGenerator
>>> StringGenerator('[\l\d]{4:18}&[\d]&[\p]').render()

The template is a string that is a sequence of one or more of the following:

  • Literal text (for example: UID)

  • Character class (for example: [a-z\s])

  • Group, a combination of literals and character classes, possibly separated by operators and using parentheses where appropriate (for example: (UID[\d]{4}&[\w]{4}))

In more detail:

Literal: <any string>

Any literal string.



Special characters need to be escaped with backslash \.

Character class: [<class specification>]

Much like in regular expressions, it uses strings of characters and hyphen for defining a class of characters.



The generator will randomly choose characters from the set of lower case letters, digits and the underscore. The number of characters generated will be exactly one in this case. For more, use a quantifier:


As a shortcut for commonly used character sets, a character set code may be used. The following will render in exactly the same way:


Character Set Codes

  • \W: whitespace + punctuation

  • \a: ascii_letters

  • \c: lowercase

  • \d: digits

  • \h: hexdigits

  • \l: letters

  • \o: octdigits

  • \p: punctuation

  • \r: printable

  • \s: whitespace

  • \u: uppercase

  • \w: _ + letters + digits

Quantifier: {x:y}

Where x is lower bound and y is upper bound. This construct must always follow immediately a class with no intervening whitespace. It is possible to write {:y} as a shorthand for {0:y} or {y} to indicate a fixed length.



Generates a string from zero to 8 in length composed of lower case alphabetic characters.


Generates a string with either four lower case alphabetic characters or a string of digits that is four in length.

Using a character class and no quantifier will result in a quantifier of 1. Thus:


will result always in either a, b, or c.

Group: (<group specification>)

A group specification is a collection of literals, character classes or other groups divided by the OR operator | or the shuffle operator &.

OR Operator

The binary | operator can be used in a group to cause one of the operands to be returned and the other to be ignored with an even chance.

Shuffle Operator

The binary & operator causes its operands to be combined and shuffled. This addresses the use case for many password requirements, such as, “at least 6 characters where 2 or more are digits”. For instance:


If a literal or a group is an operand of the shuffle operator, it will have its character sequence shuffled with the other operand.


will produce strings like:


Concatenation and Operators

Classes, literals and groups in sequence are concatenated in the order they occur. Use of the | or & operators always binds the operands immediately to the left and right:


produces something like:


In other words, the digits occur first in sequence as expected. This is equivalent to this:


Special Characters, Escaping and Errors

There are fewer special characters than regular expressions:


They can be used as literals by escaping with backslash. All other characters are treated as literals. The hyphen is only special in a character class, when it appears within square brackets.

One special case is the escape character itself, backslash ‘’. To escape this, you will need at least two backslashes to escape it. So, three alltogether: one for Python’s string interpretation and one for StringGenerator’s escaping. If for some exotic reason you want two literal backslashes in a row, you need a total of eight backslashes. The foregoing presupposes the template is a string in a file. If you are using the template in a shell command line or shell script, you’ll need to make any changes required by your specific shell.

The template parser tries to raise exceptions when syntax errors are made, but not every error will be caught, like having space between a class and quantifier.


Do not use any spaces in the template unless you intend to use them as characters in the output:

>>> SG('(zzz & yyy)').render()
u'zzyz y y'

Character Classes and Quantifiers

Use a colon in the curly braces to indicate a range. There are sensible defaults:

[\w]       # randomly choose a single word character
[\w]{0:8}  # generate word characters from 0-8 in length
[\w]{:8}   # a synonym for the above
[\w]{8}    # generate word characters of exactly 8 in length
[a-z0-9]   # generate a-z and digits, just one as there is no quantifier
[a-z0-9_!@]  # you can combine ranges with individual characters

As of version 0.1.7, quantifier ranges can alternatively be specified with a hyphen:


Here’s an example of generating a syntactically valid but, hopefully, spurious email address:


The first name will be exactly 10 lower case characters; the last name will be 5-10 characters of lower case letters, each separated by either a dot or underscore. The domain name without domain class will be 3 - 12 lower case characters and the domain type will be one of ‘.com’,’.net’,’.org’.

The following will produce strings that tend to have more letters, because the set of letters (52) is larger than the set of digits (10):


Using multiple character set codes repeatedly will increase the probability of a character from that set occuring in the result string:


This will provide a string that is three times more likely to contain a digit than the previous example.


When using the unique=True flag in the render_list() method, it’s possible the generator cannot possibly produce the required number of unique strings. For instance:

StringGenerator("[0-1]").render_list(100, unique=True)

This will generate an exception but not before attempting to generate the strings.

The number of times the generator needs to render new strings to satisfy the list length and uniqueness is not determined at parse time. The maximum number of times it will try is by default n x 10 where n is the requested length of the list. Therefore, taking the above example, the generator will attempt to generate the unique list of 0’s and 1’s 100 x 10 = 1000 times before giving up.


Unicode is supported for both the template and output.

Character Sets

Character sets used for backslashed character codes are exactly the Python character sets from the string package. While the module is designed to work on pre- Python 3, we use only those member variables from the string module that are present in Python 3. This avoids the locale-dependent sets of characters in Python 2.x.

Randomness Methods

The generator tries to use random.SystemRandom() for randint, shuffle, etc. It falls back to random.randint and associated methods if it can’t use SystemRandom.


Call the dump() method on the class instance to get useful information:

  • Version of strgen module

  • Version of Python

  • The class name used for random methods

  • The parse tree

  • The output from one invocation of the render() method

The output looks something like the following:

>>> SG('[\w]{8}&xyz|(zzz&yyy)').dump()
StringGenerator version: 1.1.2
Python version: 2.7.3 |EPD_free 7.3-2 (32-bit)| (default, Apr 12 2012, 11:28:34)
[GCC 4.0.1 (Apple Inc. build 5493)]
Random method provider class: SystemRandom

Rationale and Design Goals

In Python, the need to generate random strings comes up frequently and is accomplished usually (though not always) via something like the following code snippet:

import random
import string
mykey = ''.join(random.choice(string.ascii_uppercase + string.digits) for x in range(10))

This generates a string that is 10 characters made of uppercase letters and digits. Unfortunately, this solution becomes cumbersome when real-world requirements are added. Take for example, the typical requirement to generate a password: “a password shall have 6 - 20 characters of which at least one must be a digit and at least one must be a special character”. The above solution then becomes much more complicated and changing the requirements is an error-prone and unnecessarily complex task.

The equivalent using the strgen package:

from strgen import StringGenerator as SG

strgen is far more compact, flexible and feature-rich than using the standard solution:

  • It tries to use a better entropy mechanism and falls back gracefully if this is not available on the host OS.

  • The user can easily modify the specification (template) with minimal effort without the fear of introducing hard-to-test code paths.

  • It covers a broader set of use cases: unique ids, persistent unique filenames, test data, etc.

  • The template syntax is easy to learn for anyone familiar with regular expressions while being much simpler.

  • It supports unicode.

  • It works on Python 2.6, 2.7 and 3.3, 3.4 and 3.5 (not 3.2).

  • It proposes a standard way of expressing common requirements, like “a password shall have 6 - 20 characters of which at least one must be a digit and at least one must be a special character”:


This package is designed with the following goals in mind:

  • Provide an abstract template language that does not depend on a specific implementation language.

  • Reduce dependencies on other packages.

  • Keep syntax as simple as possible while being useful.

  • Provide an implementation design with associated behaviour that strikes the right balance between ease-of-implementation and ease-of-use.

  • Superficially similar to regular expressions to enable developers to quickly pick up the template syntax.

  • Support non-ASCII languages (unicode).


Released under the BSD license.


Thanks to Robert LeBlanc who caught some important errors in escaping special characters.

Original Author:

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