Argument to overwrite file #5

pysnooper/pysnooper.py

@@ -16,25 +16,30 @@ from . import pycompat
 from .tracer import Tracer
 
 
-def get_write_function(output):
+def get_write_and_truncate_functions(output):
     if output is None:
         def write(s):
             stderr = sys.stderr
             stderr.write(s)
+        truncate = None
     elif isinstance(output, (pycompat.PathLike, str)):
         def write(s):
             with open(output, 'a') as output_file:
                 output_file.write(s)
+        def truncate():
+            with open(output, 'w') as output_file:
+                pass
     else:
         assert isinstance(output, utils.WritableStream)
         def write(s):
             output.write(s)
+        truncate = None
 
-    return write
+    return (write, truncate)
 
 
 
-def snoop(output=None, variables=(), depth=1, prefix=''):
+def snoop(output=None, variables=(), depth=1, prefix='', overwrite=False):
     '''
     Snoop on the function, writing everything it's doing to stderr.
 
@@ -62,14 +67,20 @@ def snoop(output=None, variables=(), depth=1, prefix=''):
         @pysnooper.snoop(prefix='ZZZ ')
 
     '''
-    write = get_write_function(output)
+    write, truncate = get_write_and_truncate_functions(output)
-    @decorator.decorator
+    if truncate is None and overwrite:
-    def decorate(function, *args, **kwargs):
+        raise Exception("`overwrite=True` can only be used when writing "
+                        "content to file.")
+    def decorate(function):
         target_code_object = function.__code__
-        with Tracer(target_code_object=target_code_object,
+        tracer = Tracer(target_code_object=target_code_object, write=write,
-                    write=write, variables=variables,
+                        truncate=truncate, variables=variables, depth=depth,
-                    depth=depth, prefix=prefix):
+                        prefix=prefix, overwrite=overwrite)
-            return function(*args, **kwargs)
+
+        def inner(function_, *args, **kwargs):
+            with tracer:
+                return function(*args, **kwargs)
+        return decorator.decorate(function, inner)
 
     return decorate
 

pysnooper/tracer.py

@@ -119,18 +119,24 @@ def get_source_from_frame(frame):
     return source
 
 class Tracer:
-    def __init__(self, target_code_object, write, variables=(), depth=1,
+    def __init__(self, target_code_object, write, truncate, variables=(),
-                 prefix=''):
+                 depth=1, prefix='', overwrite=False):
         self.target_code_object = target_code_object
         self._write = write
+        self.truncate = truncate
         self.variables = variables
         self.frame_to_old_local_reprs = collections.defaultdict(lambda: {})
         self.frame_to_local_reprs = collections.defaultdict(lambda: {})
         self.depth = depth
         self.prefix = prefix
+        self.overwrite = overwrite
+        self._did_overwrite = False
         assert self.depth >= 1
 
     def write(self, s):
+        if self.overwrite and not self._did_overwrite:
+            self.truncate()
+            self._did_overwrite = True
         s = '{self.prefix}{s}\n'.format(**locals())
         if isinstance(s, bytes): # Python 2 compatibility
             s = s.decode()

tests/test_pysnooper.py

@@ -9,6 +9,7 @@ from python_toolbox import caching
 from python_toolbox import sys_tools
 from python_toolbox import temp_file_tools
 from pysnooper.third_party import six
+import pytest
 
 import pysnooper
 
@@ -179,7 +180,6 @@ def test_method_and_prefix():
     )
 
 def test_file_output():
-
     with temp_file_tools.create_temp_folder(prefix='pysnooper') as folder:
         path = folder / 'foo.log'
         @pysnooper.snoop(str(path))
@@ -295,3 +295,87 @@ def test_unavailable_source():
             )
         )
 
+
+def test_no_overwrite_by_default():
+    with temp_file_tools.create_temp_folder(prefix='pysnooper') as folder:
+        path = folder / 'foo.log'
+        with path.open('w') as output_file:
+            output_file.write(u'lala')
+        @pysnooper.snoop(str(path))
+        def my_function(foo):
+            x = 7
+            y = 8
+            return y + x
+        result = my_function('baba')
+        assert result == 15
+        with path.open() as output_file:
+            output = output_file.read()
+        assert output.startswith('lala')
+        shortened_output = output[4:]
+        assert_output(
+            shortened_output,
+            (
+                VariableEntry('foo', value_regex="u?'baba'"),
+                CallEntry('def my_function(foo):'),
+                LineEntry('x = 7'),
+                VariableEntry('x', '7'),
+                LineEntry('y = 8'),
+                VariableEntry('y', '8'),
+                LineEntry('return y + x'),
+                ReturnEntry('return y + x'),
+                ReturnValueEntry('15'),
+            )
+        )
+
+
+def test_overwrite():
+    with temp_file_tools.create_temp_folder(prefix='pysnooper') as folder:
+        path = folder / 'foo.log'
+        with path.open('w') as output_file:
+            output_file.write(u'lala')
+        @pysnooper.snoop(str(path), overwrite=True)
+        def my_function(foo):
+            x = 7
+            y = 8
+            return y + x
+        result = my_function('baba')
+        result = my_function('baba')
+        assert result == 15
+        with path.open() as output_file:
+            output = output_file.read()
+        assert 'lala' not in output
+        assert_output(
+            output,
+            (
+                VariableEntry('foo', value_regex="u?'baba'"),
+                CallEntry('def my_function(foo):'),
+                LineEntry('x = 7'),
+                VariableEntry('x', '7'),
+                LineEntry('y = 8'),
+                VariableEntry('y', '8'),
+                LineEntry('return y + x'),
+                ReturnEntry('return y + x'),
+                ReturnValueEntry('15'),
+
+                VariableEntry('foo', value_regex="u?'baba'"),
+                CallEntry('def my_function(foo):'),
+                LineEntry('x = 7'),
+                VariableEntry('x', '7'),
+                LineEntry('y = 8'),
+                VariableEntry('y', '8'),
+                LineEntry('return y + x'),
+                ReturnEntry('return y + x'),
+                ReturnValueEntry('15'),
+            )
+        )
+
+
+def test_error_in_overwrite_argument():
+    with temp_file_tools.create_temp_folder(prefix='pysnooper') as folder:
+        with pytest.raises(Exception, match='can only be used when writing'):
+            @pysnooper.snoop(overwrite=True)
+            def my_function(foo):
+                x = 7
+                y = 8
+                return y + x
+