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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Oneliner-izer
An Exercise in Constrained Coding
Chelsea Voss
[email protected]
May 31, 2016
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee.
Write a function f that takes in a string s and returns True only if
that string is composed of the characters 'A' and 'a'.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee.
Write a function f that takes in a string s and returns True only if
that string is composed of the characters 'A' and 'a'.
def f(s):
for char in s:
if char != 'a' and char != 'A':
return False
return True
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee.
Write a function f that takes in a string s and returns True only if
that string is composed of the characters 'A' and 'a'.
def f(s):
for char in s:
if char != 'a' and char != 'A':
return False
return True
More fun: solving these in one line.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee.
Write a function f that takes in a string s and returns True only if
that string is composed of the characters 'A' and 'a'.
def f(s):
for char in s:
if char != 'a' and char != 'A':
return False
return True
More fun: solving these in one line.
f = lambda s: False not in [char in 'Aa' for char in s]
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee.
Write a function f that takes in a string s and returns True only if
that string is composed of the characters 'A' and 'a'.
def f(s):
for char in s:
if char != 'a' and char != 'A':
return False
return True
More fun: solving these in one line.
f = lambda s: False not in [char in 'Aa' for char in s]
f = lambda s: all([char in 'Aa' for char in s])
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee (2).
Estimate π by sampling 100000 random points in the square [0,1]
× [0,1] and determining whether they lie in the unit circle centered
at (0, 0).
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee (2).
Estimate π by sampling 100000 random points in the square [0,1]
× [0,1] and determining whether they lie in the unit circle centered
at (0, 0).
def pi():
...
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee (2).
Estimate π by sampling 100000 random points in the square [0,1]
× [0,1] and determining whether they lie in the unit circle centered
at (0, 0).
def pi():
...
...or...
pi = lambda: sum(1 for t in xrange(100000) if
math.sqrt(random.random()**2 + random.random()**2)
<= 1) * 4.0 / 100000
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Writing one-liners
Python Bee (2).
Estimate π by sampling 100000 random points in the square [0,1]
× [0,1] and determining whether they lie in the unit circle centered
at (0, 0).
def pi():
...
...or...
pi = lambda: sum(1 for t in xrange(100000) if
math.sqrt(random.random()**2 + random.random()**2)
<= 1) * 4.0 / 100000
...Can we rewrite any Python code as a one-liner?
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Can we rewrite any Python code as a one-liner?
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Can we rewrite any Python code as a one-liner?
Technically, yes.
x = MyClass(47)
result = x.method()
print result
[...]
→
exec "x = MyClass(47)\nresult = x.method()\n[...]"
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Can we rewrite any Python code as a one-liner?
Technically, yes.
x = MyClass(47)
result = x.method()
print result
[...]
→
exec "x = MyClass(47)\nresult = x.method()\n[...]"
x = MyClass(47); result = x.method(); print result [...]
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Can we rewrite any Python code as a one-liner?
Technically, yes.
x = MyClass(47)
result = x.method()
print result
[...]
→
exec "x = MyClass(47)\nresult = x.method()\n[...]"
x = MyClass(47); result = x.method(); print result [...]
But that’s no fun!
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
More fun: computing with Python expressions. Some tools:
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
More fun: computing with Python expressions. Some tools:
List comprehension.
>>> lst = [-2, -1, 0, 1, 2, 3, 4]
>>> [i * 10 for i in lst if i > 0]
[10, 20, 30, 40]
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
More fun: computing with Python expressions. Some tools:
List comprehension.
>>> lst = [-2, -1, 0, 1, 2, 3, 4]
>>> [i * 10 for i in lst if i > 0]
[10, 20, 30, 40]
Lambda expression.
>>> f = lambda x: x * 10
>>> f(10)
4
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Rules
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Rules
One line: no newlines.
No semicolons, either.
No exec or similar.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Rules
One line: no newlines.
No semicolons, either.
No exec or similar.
Goal: Implement any Python feature as a Python expression,
ideally by abusing lambda and list comprehensions as much as
possible.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Rules
One line: no newlines.
No semicolons, either.
No exec or similar.
Goal: Implement any Python feature as a Python expression,
ideally by abusing lambda and list comprehensions as much as
possible.
Yes, this is terrible.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Rules
One line: no newlines.
No semicolons, either.
No exec or similar.
Goal: Implement any Python feature as a Python expression,
ideally by abusing lambda and list comprehensions as much as
possible.
Yes, this is terrible.
Takeaways:
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Rules
One line: no newlines.
No semicolons, either.
No exec or similar.
Goal: Implement any Python feature as a Python expression,
ideally by abusing lambda and list comprehensions as much as
possible.
Yes, this is terrible.
Takeaways:
This challenge is solvable.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Rules
One line: no newlines.
No semicolons, either.
No exec or similar.
Goal: Implement any Python feature as a Python expression,
ideally by abusing lambda and list comprehensions as much as
possible.
Yes, this is terrible.
Takeaways:
This challenge is solvable.
Lambda calculus! Obscure Python features!
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Rules
One line: no newlines.
No semicolons, either.
No exec or similar.
Goal: Implement any Python feature as a Python expression,
ideally by abusing lambda and list comprehensions as much as
possible.
Yes, this is terrible.
Takeaways:
This challenge is solvable.
Lambda calculus! Obscure Python features!
Oneliner-izer is a compiler that implements these.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
The Challenge
Rules
One line: no newlines.
No semicolons, either.
No exec or similar.
Goal: Implement any Python feature as a Python expression,
ideally by abusing lambda and list comprehensions as much as
possible.
Yes, this is terrible.
Takeaways:
This challenge is solvable.
Lambda calculus! Obscure Python features!
Oneliner-izer is a compiler that implements these.
Not for use as a software engineering paradigm.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Overview
1 The Challenge
2 Simple Code Blocks
3 Control Flow
4 Beyond
5 Building the Compiler
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Simple Code Blocks
Convert this into a single line?
x = 1
y = x + x
z = y + y
print z + z
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Simple Code Blocks
Convert this into a single line?
x = 1
y = x + x
z = y + y
print z + z
Won’t work: (exponential blowup)
print (1 + 1) + (1 + 1) + (1 + 1) + (1 + 1)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Simple Code Blocks
Convert this into a single line?
x = 1
y = x + x
z = y + y
print z + z
Won’t work: (exponential blowup)
print (1 + 1) + (1 + 1) + (1 + 1) + (1 + 1)
Answer.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Simple Code Blocks
Convert this into a single line?
x = 1
y = x + x
z = y + y
print z + z
Won’t work: (exponential blowup)
print (1 + 1) + (1 + 1) + (1 + 1) + (1 + 1)
Answer.
print
(z + z)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Simple Code Blocks
Convert this into a single line?
x = 1
y = x + x
z = y + y
print z + z
Won’t work: (exponential blowup)
print (1 + 1) + (1 + 1) + (1 + 1) + (1 + 1)
Answer.
print
(lambda z: (z + z))(y + y)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Simple Code Blocks
Convert this into a single line?
x = 1
y = x + x
z = y + y
print z + z
Won’t work: (exponential blowup)
print (1 + 1) + (1 + 1) + (1 + 1) + (1 + 1)
Answer.
print
(lambda z: (z + z))(y + y)
Alternate method. [z + z for z in [y + y]][0]
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Simple Code Blocks
Convert this into a single line?
x = 1
y = x + x
z = y + y
print z + z
Won’t work: (exponential blowup)
print (1 + 1) + (1 + 1) + (1 + 1) + (1 + 1)
Answer.
print (lambda y:
(lambda z: (z + z))(y + y)
)(x + x)
Alternate method. [z + z for z in [y + y]][0]
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Simple Code Blocks
Convert this into a single line?
x = 1
y = x + x
z = y + y
print z + z
Won’t work: (exponential blowup)
print (1 + 1) + (1 + 1) + (1 + 1) + (1 + 1)
Answer.
print (lambda x: (lambda y:
(lambda z: (z + z))(y + y)
)(x + x))(1)
Alternate method. [z + z for z in [y + y]][0]
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about functions?
Convert this into a single line?
def f(x):
return x * 10
print f(3)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about functions?
Convert this into a single line?
def f(x):
return x * 10
print f(3)
Answer.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about functions?
Convert this into a single line?
def f(x):
return x * 10
print f(3)
Answer.
print (lambda f: f(3))(lambda x: x * 10)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about functions?
Convert this into a single line?
def f(x):
return x * 10
print f(3)
Answer.
print (lambda f: f(3))(lambda x: x * 10)
Note that this works as-is with *args and **kwargs!
lambda x, y, *args, **kwargs: ...
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about operations that don’t assign to a variable?
Suppose do something() has side effects.
Convert this into a single line?
do_something()
print 42
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about operations that don’t assign to a variable?
Suppose do something() has side effects.
Convert this into a single line?
do_something()
print 42
Answer. Since the output value of do_something() isn’t used,
we can funnel it to the unused variable _.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about operations that don’t assign to a variable?
Suppose do something() has side effects.
Convert this into a single line?
do_something()
print 42
Answer. Since the output value of do_something() isn’t used,
we can funnel it to the unused variable _.
print (lambda _: 42)(do_something())
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about operations that don’t assign to a variable?
Suppose do something() has side effects.
Convert this into a single line?
do_something()
print 42
Answer. Since the output value of do_something() isn’t used,
we can funnel it to the unused variable _.
print (lambda _: 42)(do_something())
Or:
print (do_something(), 42)[1]
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about operations that don’t assign to a variable?
Suppose do something() has side effects.
Convert this into a single line?
do_something()
print 42
Answer. Since the output value of do_something() isn’t used,
we can funnel it to the unused variable _.
print (lambda _: 42)(do_something())
Or:
print (do_something(), 42)[1]
Now we don’t have to have one print: we can define our own
print() function and use it just like do something().
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
A note on print
print 1
return 2
Python 3. print is already a function.
(lambda _: 2)(print(1)) works just fine.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
A note on print
print 1
return 2
Python 3. print is already a function.
(lambda _: 2)(print(1)) works just fine.
Python 2. (lambda _: 2)(print 1) is a syntax error. How can
we get a __print() function?
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
A note on print
print 1
return 2
Python 3. print is already a function.
(lambda _: 2)(print(1)) works just fine.
Python 2. (lambda _: 2)(print 1) is a syntax error. How can
we get a __print() function?
Won’t work: In Python 2, we could use
from __future__ import print_function. However, that’s
not a real import statement, it’s a compiler directive.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
A note on print
print 1
return 2
Python 3. print is already a function.
(lambda _: 2)(print(1)) works just fine.
Python 2. (lambda _: 2)(print 1) is a syntax error. How can
we get a __print() function?
Won’t work: In Python 2, we could use
from __future__ import print_function. However, that’s
not a real import statement, it’s a compiler directive.
Instead:
__print = __builtins__.__dict__['print']
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about classes?
class Person(object):
def __init__(self):
...
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What about classes?
class Person(object):
def __init__(self):
...
→
Person = type('Person', (object,),
{'__init__': lambda self: ...})
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Putting it all together
x = 2 + 2
def f(x):
return x * 5
print x
y = f(x)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Putting it all together
x = 2 + 2
def f(x):
return x * 5
print x
y = f(x)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Putting it all together
x = 2 + 2
def f(x):
return x * 5
print x
y = f(x)
(lambda y: None)(f(x))
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Putting it all together
x = 2 + 2
def f(x):
return x * 5
print x
y = f(x)
(lambda _:
(lambda y: None)(f(x))
)(__print(x))
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Putting it all together
x = 2 + 2
def f(x):
return x * 5
print x
y = f(x)
(lambda f:
(lambda _:
(lambda y: None)(f(x))
)(__print(x))
)(lambda x: x * 5)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Putting it all together
x = 2 + 2
def f(x):
return x * 5
print x
y = f(x)
(lambda x:
(lambda f:
(lambda _:
(lambda y: None)(f(x))
)(__print(x))
)(lambda x: x * 5)
)(2 + 2)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Putting it all together
x = 2 + 2
def f(x):
return x * 5
print x
y = f(x)
(lambda x:
(lambda f:
(lambda _:
(lambda y: None)(f(x))
)(__print(x))
)(lambda x: x * 5)
)(2 + 2)
Preserves evaluation order.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Convert this into a single line?
if boolean:
x = 5
else:
x = 10
print x * 100
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Convert this into a single line?
if boolean:
x = 5
else:
x = 10
print x * 100
Answer. Conditional expressions
( if else ), plus
continuation passing.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Convert this into a single line?
if boolean:
x = 5
else:
x = 10
print x * 100
Answer. Conditional expressions
( if else ), plus
continuation passing.
(code_block_1 if boolean
else code_block_2)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Convert this into a single line?
if boolean:
x = 5
else:
x = 10
print x * 100
Answer. Conditional expressions
( if else ), plus
continuation passing.
(code_block_1 if boolean
else code_block_2)
Code blocks:
if boolean:
x = 5
print x * 100
else:
x = 10
print x * 100
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Convert this into a single line?
if boolean:
x = 5
else:
x = 10
print x * 100
Answer. Conditional expressions
( if else ), plus
continuation passing.
(code_block_1 if boolean
else code_block_2)
Code blocks:
if boolean:
x = 5
print x * 100
else:
x = 10
print x * 100
Problem: code duplication.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Convert this into a single line?
if boolean:
x = 5
else:
x = 10
print x * 100
Answer. Conditional expressions
( if else ), plus
continuation passing.
To de-duplicate, all code after
the if/else becomes a
continuation:
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Convert this into a single line?
if boolean:
x = 5
else:
x = 10
print x * 100
Answer. Conditional expressions
( if else ), plus
continuation passing.
To de-duplicate, all code after
the if/else becomes a
continuation:
def continuation(x):
print x * 100
if boolean:
x = 5
return continuation(x)
else:
x = 10
return continuation(x)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Convert this into a single line?
if boolean:
x = 5
else:
x = 10
print x * 100
Answer. Conditional expressions
( if else ), plus
continuation passing.
Final result:
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Convert this into a single line?
if boolean:
x = 5
else:
x = 10
print x * 100
Answer. Conditional expressions
( if else ), plus
continuation passing.
Final result:
(lambda continuation:
(lambda x:
continuation(x)
)(5)
if boolean else
(lambda x:
continuation(x)
)(10)
)(lambda x: __print(x * 100))
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
if/else Statements
Final result:
(lambda continuation:
(lambda x:
continuation(x)
)(5)
if boolean else
(lambda x:
continuation(x)
)(10)
)(lambda x: __print(x * 100))
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
while Loops
Convert this into a single line?
x = 5
while x < 20:
x = x + 4
print x
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
while Loops
Convert this into a single line?
x = 5
while x < 20:
x = x + 4
print x
Answer. Conditional expressions
and continuation passing...
again!
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
while Loops
Convert this into a single line?
x = 5
while x < 20:
x = x + 4
print x
Answer. Conditional expressions
and continuation passing...
again!
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
while Loops
x = 5
while x < 20:
x = x + 4
print x
→
x = 5
def while_loop(x):
if x < 20:
x = x + 4
while_loop(x)
else:
print x
while_loop(x)
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
while Loops
x = 5
while x < 20:
x = x + 4
print x
→
x = 5
def while_loop(x):
if x < 20:
x = x + 4
while_loop(x)
else:
print x
while_loop(x)
Problem: while loop is recursive!
Not an anonymous function!
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
while Loops
x = 5
while x < 20:
x = x + 4
print x
→
x = 5
def while_loop(x):
if x < 20:
x = x + 4
while_loop(x)
else:
print x
while_loop(x)
Problem: while loop is recursive!
Not an anonymous function!
Solution: Y combinator.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
while Loops
x = 5
while x < 20:
x = x + 4
print x
→
x = 5
def while_loop(x):
if x < 20:
x = x + 4
while_loop(x)
else:
print x
while_loop(x)
Problem: while loop is recursive!
Not an anonymous function!
Solution: Y combinator.
Y =
(lambda f: (lambda x: x(x))
(lambda y: f(lambda: y(y)())))
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
while Loops
x = 5
while x < 20:
x = x + 4
print x
→
Problem: while loop is recursive!
Not an anonymous function!
Solution: Y combinator.
Y =
(lambda f: (lambda x: x(x))
(lambda y: f(lambda: y(y)())))
(lambda x: (lambda while_loop: while_loop(x))
(Y(lambda while_loop: (lambda x: (lambda x:
while_loop(x))(x+4) if x<20 else __print(x)))))(5)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
while Loops
x = 5
while x < 20:
x = x + 4
print x
→
Problem: while loop is recursive!
Not an anonymous function!
Solution: Y combinator.
Y =
(lambda f: (lambda x: x(x))
(lambda y: f(lambda: y(y)())))
(lambda x: (lambda while_loop: while_loop(x))
(Y(lambda while_loop: (lambda x: (lambda x:
while_loop(x))(x+4) if x<20 else __print(x)))))(5)
Worked example here:
Wikipedia:Fixed-point_combinator#The_factorial_function
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Storing state
Old way:
(lambda x: return_value)(42)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Storing state
Old way:
(lambda x: return_value)(42)
Problem:
continuation = (lambda x, y, z, kitchen_sink: ...)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Storing state
Old way:
(lambda x: return_value)(42)
Problem:
continuation = (lambda x, y, z, kitchen_sink: ...)
New way:
[return_value for some_dict['x'] in [42]][0]
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Storing state
Old way:
(lambda x: return_value)(42)
Problem:
continuation = (lambda x, y, z, kitchen_sink: ...)
New way:
[return_value for some_dict['x'] in [42]][0]
More concise continuations:
continuation = (lambda some_dict: ...)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Storing state
Old way:
(lambda x: return_value)(42)
Problem:
continuation = (lambda x, y, z, kitchen_sink: ...)
New way:
[return_value for some_dict['x'] in [42]][0]
More concise continuations:
continuation = (lambda some_dict: ...)
Initialize some dict with locals().
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Storing state
Old way:
(lambda x: return_value)(42)
Problem:
continuation = (lambda x, y, z, kitchen_sink: ...)
New way:
[return_value for some_dict['x'] in [42]][0]
More concise continuations:
continuation = (lambda some_dict: ...)
Initialize some dict with locals().
Bonus: now we can import x from one-lined programs!
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
for Loops
Convert this into a single
line?
total = 0
for item in iterable:
total += item
print total
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
for Loops
Convert this into a single
line?
total = 0
for item in iterable:
total += item
print total
Reminder.
The items of iterable must
be consumed one-by-one in
order. We can’t always index
into it with iterable[i].
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
for Loops
Convert this into a single
line?
total = 0
for item in iterable:
total += item
print total
Reminder.
The items of iterable must
be consumed one-by-one in
order. We can’t always index
into it with iterable[i].
>>> iterable = {10, 20, 30}
>>> for item in iterable:
... print item
...
10
20
30
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
for Loops
Convert this into a single
line?
total = 0
for item in iterable:
total += item
print total
Reminder.
The items of iterable must
be consumed one-by-one in
order. We can’t always index
into it with iterable[i].
>>> iterable = {10, 20, 30}
>>> for item in iterable:
... print item
...
10
20
30
>>> iterable[2]
TypeError: 'set' object
does not support indexing
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
for Loops
Convert this into a single
line?
total = 0
for item in iterable:
total += item
print total
Answer.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
for Loops
Convert this into a single
line?
total = 0
for item in iterable:
total += item
print total
Answer.
Convert to a while loop that
consumes the iterable using
next.
total = 0
items = iter(iterable)
sentinel = []
while True:
item = next(items, sentinel)
if item is sentinel:
break
total += item
print total
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Imports
import random as rnd
print rnd.choice([1, 2, 3, 10])
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Imports
import random as rnd
print rnd.choice([1, 2, 3, 10])
Answer. This is equivalent to:
rnd = __import__('random')
print rnd.choice([1, 2, 3, 10])
Fortunately, __import__ itself doesn’t need to be imported.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Raising Errors
raise Bad()
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Raising Errors
raise Bad()
→
([] for [] in []).throw(Bad())
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Raising Errors
raise Bad()
→
([] for [] in []).throw(Bad())
assert good
carry_on()
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Raising Errors
raise Bad()
→
([] for [] in []).throw(Bad())
assert good
carry_on()
→
carry_on() if good else
([] for [] in []).throw(AssertionError())
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
Problem:
try:
foo()
except Bad as ev:
bar(ev)
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
Problem:
try:
foo()
except Bad as ev:
bar(ev)
Solution: abuse the context manager protocol!
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
Solution: abuse the context manager protocol!
class Handler:
def __enter__(self): pass
def __exit__(self, et, ev, tb):
if et is not None and issubclass(et, Bad):
bar(ev); return True
return False
with Handler():
foo()
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
class Body:
def __enter__(self): pass
def __exit__(self, et, ev, tb):
foo()
class Handler:
def __enter__(self): pass
def __exit__(self, et, ev, tb):
if et is not None and issubclass(et, Bad):
bar(ev); return True
return False
with Handler(), Body():
pass
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
class Body:
def __enter__(self): pass
def __exit__(self, et, ev, tb):
foo()
class Handler:
def __enter__(self): pass
def __exit__(self, et, ev, tb):
if et is not None and issubclass(et, Bad):
bar(ev); return True
return False
with contextlib.nested(Handler(), Body()):
pass
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
class Body:
def __enter__(self): pass
def __exit__(self, et, ev, tb):
foo() # Why __exit__? Python issue 5251.
class Handler:
def __enter__(self): pass
def __exit__(self, et, ev, tb):
if et is not None and issubclass(et, Bad):
bar(ev); return True
return False
with contextlib.nested(Handler(), Body()):
pass
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
class Body:
def __enter__(self): pass
def __exit__(self, et, ev, tb):
foo() # Why __exit__? Python issue 5251.
class Handler:
def __enter__(self): pass
def __exit__(self, et, ev, tb):
if et is not None and issubclass(et, Bad):
bar(ev); return True
return False
ctx = contextlib.nested(Handler(), Body())
ctx.__enter__()
ctx.__exit__(None, None, None)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
Body = type(’Body’, (),
’__enter__’: lambda self: None,
’__exit__’: lambda self, et, ev, tb: foo()
)
Handler = type(’Handler’, (),
’__enter__’: lambda self: None,
’__exit__’: lambda self, et, ev, tb:
et is not None and issubclass(et, Bad) and
(bar(ev), True)[1]
)
ctx = contextlib.nested(Handler(), Body())
ctx.__enter__()
ctx.__exit__(None, None, None)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
(lambda ctx:
(ctx.__enter__(), ctx.__exit__(None, None, None))
)(contextlib.nested(
type(’Handler’, (), {
’__enter__’: lambda self: None,
’__exit__’: lambda self, et, ev, tb:
et is not None and issubclass(et, Bad) and
(bar(ev), True)[1]
})(), type(’Body’, (), {
’__enter__’: lambda self: None,
’__exit__’: lambda self, et, ev, tb: foo()
})()))
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
try/except
(lambda ctx:
(ctx.__enter__(), ctx.__exit__(None, None, None))
)(contextlib.nested(
type(’Handler’, (), {
’__enter__’: lambda self: None,
’__exit__’: lambda self, et, ev, tb:
et is not None and issubclass(et, Bad) and
(bar(ev), True)[1]
})(), type(’Body’, (), {
’__enter__’: lambda self: None,
’__exit__’: lambda self, et, ev, tb: foo()
})()))
Also implemented: else and finally.
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
What’s Left
from module import *
yield and generators
with
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Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Building the Compiler
ast - for parsing Python files into syntax trees
symtable - for elucidating the scope of variables
argparse - for parsing command-line arguments
unittest - test suite
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Building the Compiler
ast - for parsing Python files into syntax trees
symtable - for elucidating the scope of variables
argparse - for parsing command-line arguments
unittest - test suite
https://github.com/csvoss/onelinerizer
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Some caveats
Constant upper limit to Python parser.
$ python onelinerized.py
s_push: parser stack overflow
MemoryError
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Some caveats
Constant upper limit to Python parser.
$ python onelinerized.py
s_push: parser stack overflow
MemoryError
$ pypy onelinerized.py
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Some caveats
Constant upper limit to Python parser.
$ python onelinerized.py
s_push: parser stack overflow
MemoryError
$ pypy onelinerized.py
Long loops: Maximum recursion depth exceeded.
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Some caveats
Constant upper limit to Python parser.
$ python onelinerized.py
s_push: parser stack overflow
MemoryError
$ pypy onelinerized.py
Long loops: Maximum recursion depth exceeded.
import sys
sys.setrecursionlimit(new_limit)
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Oneliner-izing the Oneliner-izer
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Contributors
Many thanks to:
andersk for contributing many features and some slides
asottile and shulinye for contributing code
Chelsea Voss PyCon 2016
Oneliner-izer
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The Challenge Simple Code Blocks Control Flow Beyond Building the Compiler
Links
Demo: http://onelinepy.herokuapp.com/
Code: https://github.com/csvoss/onelinerizer
Further Reading
Lambda calculus, Church numerals, combinatory logic
To Mock a Mockingbird for logic and combinator puzzles
Chelsea Voss PyCon 2016
Oneliner-izer