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37 lines
2.1 KiB
Markdown
37 lines
2.1 KiB
Markdown
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# String Module Cheat Sheet
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## TEMPLATE STRINGS
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Template strings support $-based substitutions, using the following rules:
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`$$` is an escape; it is replaced with a single `$`.
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`$identifier` names a substitution placeholder matching a mapping key of "identifier".
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By default, "identifier" is restricted to any case-insensitive ASCII alphanumeric string (including underscores) that starts with an underscore or ASCII letter.
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The first non-identifier character after the $ character terminates this placeholder specification.
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`${identifier}` is equivalent to `$identifier`.
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It is required when valid identifier characters follow the placeholder but are not part of the placeholder.
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Any other appearance of `$` in the string will result in a `ValueError` being raised.
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The string module provides a Template class that implements these rules.
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```python
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from string import Template
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# The methods of Template are:
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string.Template(template) # The constructor takes a single argument which is the template string.
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substitute(mapping={}, **kwargs)
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# Performs the template substitution, returning a new string.
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# mapping is any dictionary-like object with keys that match the placeholders in the template.
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# Alternatively, you can provide keyword arguments, where the keywords are the placeholders.
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# When both mapping and kwds are given and there are duplicates, the placeholders from kwds take precedence.
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safe_substitute(mapping={}, **kwargs)
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# Like substitute(), except that if placeholders are missing from mapping and kwds,
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# instead of raising a KeyError exception, the original placeholder will appear in the resulting string intact.
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# Also, unlike with substitute(), any other appearances of the $ will simply return $ instead of raising ValueError.
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# While other exceptions may still occur, this method is called “safe” because it always tries to return a usable string instead of raising an exception.
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# In another sense, safe_substitute() may be anything other than safe, since it will silently ignore malformed templates containing dangling delimiters, unmatched braces, or placeholders that are not valid Python identifiers.
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```
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