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Tips for golfing in Python

What general tips do you have for golfing in Python? I'm looking for ideas which can be applied to code-golf problems and which are also at least somewhat specific to Python (e.g. "remove comments" is not an answer).

Please post one tip per answer.

Answer*

## Length tradeoff reference



I've think it would be useful to have a reference for the character count differences for some common alternative ways of doing things, so that I can know when to use which. I'll use `_` to indicate an expression or piece of code.

**Assign to a variable: +4**

    x=_;x
    _

So, this breaks even if you

 - Use `_` a second time: `_` has length 5
 - Use `_` a third time: `_` has length 3

**Assign variables separately: 0**

    x,y=a,b
    x=a;y=b
 
 - -2 when `a` equals `b` for `x=y=a`

**Expand `lambda` to function `def`: +7**

    lambda x:_
    def f(x):return _

 - -2 for named functions
 - -1 if `_` can touch on the left
 - -1 in Python 2 if can `print` rather than return
 - +1 for starred input `*x` 

Generically, if you're `def` to save an expression to a variable used twice, this breaks even when the expression is length 12.

    lambda x:g(123456789012,123456789012)
    def f(x):s=123456789012;return g(s,s)

**STDIN rather than function: +1**

    def f(x):_;print s
    x=input();_;print s

 - -1 for line of code needed in `_` if not single-line
 - +4 if `raw_input` needed in Python 2
 - -4 if input variable used only once
 - +1 if function must `return` rather than `print` in Python 2

**Use `exec` rather than looping over `range(n)`: +0**

    for i in range(n):_
    i=0;exec"_;i+=1;"*n
    
 - +2 for Python 3 `exec()`
 - -4 if shifted range `range(c,c+n)` for single-char `c`
 - -5 when going backwards from `n` to `1` via `range(n,0,-1)`
 - -9 if index variable never used

**Apply `map` manually in a loop: +0**

    for x in l:y=f(x);_
    for y in map(f,l):_

**Apply `map` manually in a list comprehension: +8**

    map(f,l)
    [f(x)for x in l]

 - -12 when `f` must be written in the `map` as the `lambda` expression `lambda x:f(x)`, causing overall 4 char loss.

**Apply `filter` manually in a list comprehension: +11**

    filter(f,l)
    [x for x in l if f(x)]

 - -1 if `f(x)` expression can touch on the left
 - -12 when `f` must be written in the `filter` as the `lambda` expression `lambda x:f(x)`, causing overall 1 char loss.

**Import* versus import single-use: +4** 

    import _;_.f
    from _ import*;f

 - Breaks even when `_` has length 5
 - `import _ as x;x.f` is always worse except for multiple imports
 - `__import__('_').f` is also worse

Thanks to @Sp3000 for lots of suggestions and fixes.

If this is an answer to a challenge…

…Be sure to follow the challenge specification. However, please refrain from exploiting obvious loopholes. Answers abusing any of the standard loopholes are considered invalid. If you think a specification is unclear or underspecified, comment on the question instead.
…Try to optimize your score. For instance, answers to code-golf challenges should attempt to be as short as possible. You can always include a readable version of the code in addition to the competitive one. Explanations of your answer make it more interesting to read and are very much encouraged.
…Include a short header which indicates the language(s) of your code and its score, as defined by the challenge.

More generally…

…Please make sure to answer the question and provide sufficient detail.
…Avoid asking for help, clarification or responding to other answers (use comments instead).

Edit Summary*

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4

\$\begingroup\$ The "exec" bit goes -9 if you don't need the index. \$\endgroup\$

T. Verron
– T. Verron

2015-10-08 08:43:20 +00:00
Commented Oct 8, 2015 at 8:43
1

\$\begingroup\$ lambda x:g(123456789012,123456789012) can be replaced with lambda x:g(a:=123456789012,a) \$\endgroup\$

Lucenaposition
– Lucenaposition

2024-08-19 00:10:03 +00:00
Commented Aug 19, 2024 at 0:10

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Correct minor typos or mistakes
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create code fences with backticks ` or tildes ~
```
like so
```
add language identifier to highlight code
```python
def function(foo):
print(foo)
```
put returns between paragraphs
for linebreak add 2 spaces at end
_italic_ or **bold**
indent code by 4 spaces
backtick escapes `like _so_`
quote by placing > at start of line
to make links (use https whenever possible)

<https://example.com>

[example](https://example.com)

<a href="/api/flow.js?q=https%3A%2F%2Fexample.com">example</a>
MathJax equations \$\sin^2 \theta\$

formatting help »
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