Python

https://www.python.org/doc/
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Python is a versatile, high-level programming language designed for readability and used extensively...
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### Sample run of find_path

Source: https://www.python.org/doc/essays/graphs

Interactive Python session showing the output of find_path(graph, 'A', 'D') using the example graph.

```python
    >>> find_path(graph, 'A', 'D')
    ['A', 'B', 'C', 'D']
    >>>
```

--------------------------------

### Integer for-loop example using range

Source: https://www.python.org/doc/essays/pepparade

Shows the traditional for loop with range(10) that PEP 284 aims to replace.

```python
for i in range(10):
```

--------------------------------

### Tracing metaclass example

Source: https://www.python.org/doc/essays/metaclasses

A complete simplified example of a metaclass that automatically inserts trace calls for all method calls. It defines the Tracing metaclass, the Instance class, and the BoundMethod class, then creates a class MyTracedClass and demonstrates tracing on method calls.

```python
import types

class Tracing:
    def __init__(self, name, bases, namespace):
        """Create a new class."""
        self.__name__ = name
        self.__bases__ = bases
        self.__namespace__ = namespace
    def __call__(self):
        """Create a new instance."""
        return Instance(self)

class Instance:
    def __init__(self, klass):
        self.__klass__ = klass
    def __getattr__(self, name):
        try:
            value = self.__klass__.__namespace__[name]
        except KeyError:
            raise AttributeError, name
        if type(value) is not types.FunctionType:
            return value
        return BoundMethod(value, self)

class BoundMethod:
    def __init__(self, function, instance):
        self.function = function
        self.instance = instance
    def __call__(self, *args):
        print "calling", self.function, "for", self.instance, "with", args
        return apply(self.function, (self.instance,) + args)

Trace = Tracing('Trace', (), {})

class MyTracedClass(Trace):
    def method1(self, a):
        self.a = a
    def method2(self):
        return self.a

aninstance = MyTracedClass()

aninstance.method1(10)

print "the answer is %d" % aninstance.method2()
```

--------------------------------

### Define __all__ in Sounds/Effects/__init__.py

Source: https://www.python.org/doc/essays/packages

Defines the list of module names to be imported when 'from Sound.Effects import *' is used. This example imports the three named submodules.

```python
__all__ = ["echo", "surround", "reverse"]
```

--------------------------------

### Example usage of iterindexed with start and stop arguments

Source: https://www.python.org/doc/essays/pepparade

Shows a hypothetical call to iterindexed with start=5 and stop=10, printing indices 5 through 9. The author notes this would be confusing because the spec would print the first five characters, not the range.

```python
for i, j in iterindexed("abcdefghij", 5, 10): print i, j
```

--------------------------------

### Proposed iterindexed implementation with start and stop

Source: https://www.python.org/doc/essays/pepparade

A proposed implementation of iterindexed that accepts start and stop arguments, iterating over sequence indices and yielding (i, item) pairs. The author suggests this reduces validity to sequences only but makes iterindexed(x, i, j) iterate over x[i:j].

```python
def iterindexed(sequence, start=0, stop=None):
    i = start
    while stop is None or i < stop:
        try:
            item = sequence[i]
        except IndexError:
            break
        yield (i, item)
        i += 1
```

--------------------------------

### Enum metaclass example: defining enumerated types with class syntax

Source: https://www.python.org/doc/essays/metaclasses

This example defines an enumerated type using a metaclass called Enum. The class attributes are the enumerated values, and instances are never created. Accessing an attribute like Color.red returns a string representation, equality with the integer value works, and arithmetic operations raise a TypeError.

```python
class Color(Enum):
    red = 1
    green = 2
    blue = 3
print Color.red
```

--------------------------------

### Create instances of MySpecialClass and print their class

Source: https://www.python.org/doc/essays/metaclasses

Creates two instances of MySpecialClass and prints their __class__ attribute, which shows they are instances of HelperClass1.

```python
x = MySpecialClass()
y = MySpecialClass()
print x.__class__, y.__class__
```

--------------------------------

### Package directory structure for sound handling

Source: https://www.python.org/doc/essays/packages

Shows the hierarchical filesystem layout of the Sound package with subpackages Utils, Formats, Effects, and Filters, each containing __init__.py and module files.

```text
Sound/				Top-level package
      __init__.py		Initialize the sound package
      Utils/			Subpackage for internal use
            __init__.py
            iobuffer.py
	    errors.py
	    ...
      Formats/			Subpackage for file format conversions
              __init__.py
              wavread.py
	      wavwrite.py
	      aiffread.py
	      aiffwrite.py
	      auread.py
	      auwrite.py
	      ...
      Effects/			Subpackage for sound effects
              __init__.py
	      echo.py
	      surround.py
	      reverse.py
	      ...
      Filters/			Subpackage for filters
              __init__.py
              equalizer.py
	      vocoder.py
	      karaoke.py
	      dolby.py
	      ...

```

--------------------------------

### Directory structure for platform-specific effects

Source: https://www.python.org/doc/essays/packages

Shows the directory layout for a package with platform-specific submodules. The Effects/__init__.py can manipulate __path__ to prioritize platform-specific implementations.

```text
Sound/
      __init__.py
      Effects/			# Generic versions of effects modules
              __init__.py
              echo.py
	      surround.py
	      reverse.py
	      ...
              plat-ix86/	# Intel x86 specific effects modules
	                echo.py
			surround.py
	      plat-PPC/		# PPC specific effects modules
	                echo.py

```

--------------------------------

### Variable lower bound in integer for-loop

Source: https://www.python.org/doc/essays/pepparade

An example with a variable lower bound, showing potential confusion with tuple unpacking syntax.

```python
for i <= j < k:
```

--------------------------------

### Import echo with from Sound.Effects import echo

Source: https://www.python.org/doc/essays/packages

Loads the submodule echo and makes it available without the package prefix, so it can be used as echo.echofilter(input, output, delay=0.7, atten=4).

```python
from Sound.Effects import echo
```

--------------------------------

### Graph arc list

Source: https://www.python.org/doc/essays/graphs

A textual representation of a directed graph with six nodes (A-F) and eight arcs, used as the example graph throughout the essay.

```text
    A -> B
    A -> C
    B -> C
    B -> D
    C -> D
    D -> C
    E -> F
    F -> C
```

--------------------------------

### f6: Using string.joinfields and map for speed

Source: https://www.python.org/doc/essays/list2str

The fastest version, using implied loops and C-implemented string concatenation. Runs four to five times faster than f3 and avoids quadratic behavior.

```python
    import string
    def f6(list):
        return string.joinfields(map(chr, list), "")

```

--------------------------------

### Extracting message and info from SyntaxError

Source: https://www.python.org/doc/essays/stdexceptions

The same approach works for SyntaxError, but the info part is not always present. The example uses a nested try to handle missing info.

```python
try:
    c = compile(source, filename, "exec")
except SyntaxError, what:
    try:
        message, info = what
    except:
        message, info = what, None
    if info:
        "...print source code info..."
    print "SyntaxError:", msg
```
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