PromptTemplate Silently Accepts Empty F-String Field `{}` Then Crashes With IndexError On Format
PromptTemplate.from_template("Value: {}") is accepted silently, reports an empty input variable, and then raises IndexError on every .format() call. It should be rejected at creation time with a clear ValueError, consistent with the all-digit `{0}` case.
The f-string validation in langchain_core.prompts.string only checks for all-digit variable names. An empty replacement field `{}` becomes a variable named `''`, which passes the all-digit check because an empty string is not all digits. Later, Python's string.Formatter interprets `{}` as an auto-numbered positional field and calls get_value with key `''`, but the positional args tuple contains no matching element, causing IndexError.
```python
from langchain_core.prompts import PromptTemplate
t = PromptTemplate.from_template("Value: {}")
print(t.input_variables) # ['']
t.format() # IndexError: tuple index out of range
t.format(**{"": "x"}) # IndexError: tuple index out of range
```
Fixing Code Block
def validate_f_string_template(template: str, *, allowed_input_variables: list[str] | None = None) -> set[str]:
"""Validate f-string template variables."""
try:
variables = get_template_variables(template, "f-string")
except ValueError as e:
raise ValueError(f"Invalid f-string: {e}") from e
for variable in variables:
if variable == "":
raise ValueError(
"Variable names cannot be empty. Use '{{}}' to escape literal braces."
)
if variable.isdigit():
raise ValueError(
"Variable names cannot be all digits; they are interpreted as positional arguments."
)
if allowed_input_variables is not None:
undefined = variables - set(allowed_input_variables)
if undefined:
raise ValueError(
f"Undefined variables: {sorted(undefined)}"
)
return variables
After extracting variables from the f-string template, the function now rejects any variable whose name is an empty string, raising a clear ValueError at template creation time. This prevents the invalid `{}` from being stored and later crashing with an opaque IndexError. The existing all-digit check is retained.
Edge Case Audit
This fix changes behavior for templates containing `{}`. Previously they were accepted and only failed when formatting was attempted; after the fix they raise ValueError during PromptTemplate.from_template(). This is the intended safe behavior, but any application that intentionally created such templates (however unlikely) will break. To rollback, revert to the previous function or temporarily remove the empty-string check. Users who need literal braces should escape them as `{{}}`. The validation is pure and has no concurrency or threading implications. Apply this patch to langchain-core 1.6.3 and current master after running existing prompt tests.
