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Python uses the following syntax to express list comprehensions over finite lists: S = [ 2 * x for x in range ( 100 ) if x ** 2 > 3 ] A generator expression may be used in Python versions >= 2.4 which gives lazy evaluation over its input, and can be used with generators to iterate over 'infinite' input such as the count generator function which ...
Here, the list [0..] represents , x^2>3 represents the predicate, and 2*x represents the output expression.. List comprehensions give results in a defined order (unlike the members of sets); and list comprehensions may generate the members of a list in order, rather than produce the entirety of the list thus allowing, for example, the previous Haskell definition of the members of an infinite list.
A language that supports the statement construct typically has rules for one or more of the following aspects: Statement terminator – marks the end of a statement; Statement separator – demarcates the boundary between two statements; need needed for the last statement; Line continuation – escapes a newline to continue a statement on the ...
In Python, the set-builder's braces are replaced with square brackets, parentheses, or curly braces, giving list, generator, and set objects, respectively. Python uses an English-based syntax. Haskell replaces the set-builder's braces with square brackets and uses symbols, including the standard set-builder vertical bar.
The list comprehension will immediately create a large list (with 78498 items, in the example, but transiently creating a list of primes under two million), even if most elements are never accessed. The generator comprehension is more parsimonious.
The literature on programming languages contains an abundance of informal claims about their relative expressive power, but there is no framework for formalizing such statements nor for deriving interesting consequences. [52] This table provides two measures of expressiveness from two different sources.
For brevity, these words will have the specified meanings in the following tables (unless noted to be part of language syntax): funcN A function.
# This function has no guard clause def f_noguard (x): if isinstance (x, int): #code #code #code return x + 1 else: return None # Equivalent function with a guard clause. Note that most of the code is less indented, which makes it easier to read and reason about def f_guard (x): if not isinstance (x, int): return None #code #code #code return x + 1