Add Escape Curly Braces Within Formatted String as a Python TIL

python/escape-curly-braces-within-formatted-string.md

@@ -0,0 +1,47 @@
+# Escape Curly Braces Within Formatted String
+
+Python's
+[_f-string_](https://docs.python.org/3/reference/lexical_analysis.html#f-strings)
+allows you to interpolate values into a string literal like so:
+
+```python
+>>> amount = 25
+>>> print(f"Price: ${amount}")
+Price: $25
+```
+
+The `{` and `}` are betting special treatment when `f` precedes the string
+literal. So, what if the resulting string is supposed to have literal `{` and
+`}` characters in it?
+
+```python
+>>> f"JSON opens and closes with curlies: {...}"
+'JSON opens and closes with curlies: Ellipsis'
+```
+
+I need a way of escaping them. The trick is to double them up.
+
+```python
+>>> f"JSON opens and closes with curlies: {{...}}"
+'JSON opens and closes with curlies: {...}'
+```
+
+A more practical example of where this is useful is in formatting an expected
+string of JSON in a test:
+
+```python
+info_result = runner.invoke(cli, ["info", "--json"])
+output = textwrap.dedent(f"""
+{{
+    "config_file": "{config_file}",
+    "data_dir": "{data_dir}",
+    "version": "{VMT_VERSION}",
+    "schema_version": 1
+}}
+""").strip()
+assert output in info_result.output
+```
+
+I'm doing some interpolation at various points in the string literal, but the
+outer curly braces are meant to be literal, the opening and closing of the JSON
+object.